nrf: Change selected boards to utilize pre-flashed bootloader.

The nrf52840-mdk-usb-dongle and pca10050 comes with a pre-flashed
bootloader (OpenBootloader).

This commit updates the boards "mpconfigboard.mk" to use DFU as
default flashing method and set the corresponding BOOTLOADER
settings such that nrf52840_open_bootloader_1.2.x.ld linker
script is used.

The default DFU flashing method can be disabled by issuing "DFU=0"
when invoking make. This will lead to "segger" being used as default
flashing tool. When using "DFU=0", the linker scripts will not
compensate for any MBR and Bootloader region being present, and might
overwrite them if they were present.

The commit also removes the custom linker script specific to
nrf52840-mdk-usb-dongle as it now points to a generic.

Updated nrf52840-mdk-usb-dongle's README.md to be more clear on
how to deploy the built firmware.

The port README.md has also been updated. In the list of target
boards a new column has been added to indicate which bootloader
is present on the target board. And for consistency, changed all
examples in the README.md to use "deploy" instead of "flash".

5 files changed, 77 insertions, 30 deletions

diff --git a/ports/nrf/README.md b/ports/nrf/README.md
index 5fceb6705..22bb4af51 100644
--- a/ports/nrf/README.md
+++ b/ports/nrf/README.md
@@ -58,12 +58,12 @@ By default, the PCA10040 (nrf52832) is used as compile target. To build and flas
 
     make submodules
     make
-    make flash
+    make deploy
 
 Alternatively the target board could be defined:
 
      make BOARD=pca10040
-     make BOARD=pca10040 flash
+     make BOARD=pca10040 deploy
 
 ## Compile without LTO enabled
 
@@ -118,26 +118,27 @@ For example:
 
 ## Target Boards and Make Flags
 
-Target Board (BOARD) | Bluetooth Stack (SD)    | Bluetooth Support      | Flash Util
----------------------|-------------------------|------------------------|-------------------------------
-microbit             | s110                    | Peripheral             | [PyOCD](#pyocdopenocd-targets)
-pca10000             | s110                    | Peripheral             | [Segger](#segger-targets)
-pca10001             | s110                    | Peripheral             | [Segger](#segger-targets)
-pca10028             | s110                    | Peripheral             | [Segger](#segger-targets)
-pca10031             | s110                    | Peripheral             | [Segger](#segger-targets)
-wt51822_s4at         | s110                    | Peripheral             | Manual, see [datasheet](https://4tronix.co.uk/picobot2/WT51822-S4AT.pdf) for pinout
-pca10040             | s132                    | Peripheral and Central | [Segger](#segger-targets)
-feather52            | s132                    | Peripheral and Central | Manual, SWDIO and SWCLK solder points on the bottom side of the board
-arduino_primo        | s132                    | Peripheral and Central | [PyOCD](#pyocdopenocd-targets)
-ibk_blyst_nano       | s132                    | Peripheral and Central | [IDAP](#idap-midap-link-targets)
-idk_blyst_nano       | s132                    | Peripheral and Central | [IDAP](#idap-midap-link-targets)
-blueio_tag_evim      | s132                    | Peripheral and Central | [IDAP](#idap-midap-link-targets)
-evk_nina_b1          | s132                    | Peripheral and Central | [Segger](#segger-targets)
-pca10056             | s140                    | Peripheral and Central | [Segger](#segger-targets)
-pca10059             | s140                    | Peripheral and Central | Manual, SWDIO and SWCLK solder points on the sides.
-particle_xenon       | s140                    | Peripheral and Central | [Black Magic Probe](#black-magic-probe-targets)
-pca10090             | None (bsdlib.a)         | None (LTE/GNSS)        | [Segger](#segger-targets)
-actinius_icarus      | None (bsdlib.a)         | None (LTE/GNSS)        | [Segger](#segger-targets)
+Target Board (BOARD) | Bluetooth Stack (SD)    | Bluetooth Support      | Bootloader     | Default Flash Util
+---------------------|-------------------------|------------------------|----------------|-------------------
+microbit             | s110                    | Peripheral             |                | [PyOCD](#pyocdopenocd-targets)
+pca10000             | s110                    | Peripheral             |                | [Segger](#segger-targets)
+pca10001             | s110                    | Peripheral             |                | [Segger](#segger-targets)
+pca10028             | s110                    | Peripheral             |                | [Segger](#segger-targets)
+pca10031             | s110                    | Peripheral             |                | [Segger](#segger-targets)
+wt51822_s4at         | s110                    | Peripheral             |                | Manual, see [datasheet](https://4tronix.co.uk/picobot2/WT51822-S4AT.pdf) for pinout
+pca10040             | s132                    | Peripheral and Central |                | [Segger](#segger-targets)
+feather52            | s132                    | Peripheral and Central |                | Manual, SWDIO and SWCLK solder points on the bottom side of the board
+arduino_primo        | s132                    | Peripheral and Central |                | [PyOCD](#pyocdopenocd-targets)
+ibk_blyst_nano       | s132                    | Peripheral and Central |                | [IDAP](#idap-midap-link-targets)
+idk_blyst_nano       | s132                    | Peripheral and Central |                | [IDAP](#idap-midap-link-targets)
+blueio_tag_evim      | s132                    | Peripheral and Central |                | [IDAP](#idap-midap-link-targets)
+evk_nina_b1          | s132                    | Peripheral and Central |                | [Segger](#segger-targets)
+pca10056             | s140                    | Peripheral and Central |                | [Segger](#segger-targets)
+pca10059             | s140                    | Peripheral and Central | OpenBootloader | [nrfutil](#nrfutil-targets)
+particle_xenon       | s140                    | Peripheral and Central |                | [Black Magic Probe](#black-magic-probe-targets)
+nrf52840-mdk-usb-dongle | s140                 | Peripheral and Central | OpenBootloader | [nrfutil](#nrfutil-targets)
+pca10090             | None (bsdlib.a)         | None (LTE/GNSS)        |                | [Segger](#segger-targets)
+actinius_icarus      | None (bsdlib.a)         | None (LTE/GNSS)        |                | [Segger](#segger-targets)
 
 ## IDAP-M/IDAP-Link Targets
 
@@ -173,6 +174,31 @@ This requires no further dependencies other than `arm-none-eabi-gdb`.
 [this guide](https://github.com/blacksphere/blackmagic/wiki/Useful-GDB-commands)
 for more tips about using the BMP with GDB.
 
+## nRFUtil Targets
+
+Install the necessary Python packages that will be used for flashing using the bootloader:
+
+    sudo pip install nrfutil
+    sudo pip install intelhex
+
+The `intelhex` provides the `hexmerge.py` utility which is used by the Makefile
+to trim of the MBR in case SoftDevice flashing is requested.
+
+`nrfutil` as flashing backend also requires a serial port paramter to be defined
+in addition to the `deploy` target of make. For example:
+
+    make BOARD=nrf52840-mdk-usb-dongle NRFUTIL_PORT=/dev/ttyACM0 deploy
+
+If the target device is connected to `/dev/ttyACM0` serial port, the
+`NRFUTIL_PORT` parameter to make can be elided as it is the default serial
+port set by the Makefile.
+
+When enabling Bluetooth LE, as with the other flash utils, the SoftDevice
+needs to be flashed in the first firmware update. This can be done by issuing
+the `sd` target instead of `deploy`. For example:
+
+    make BOARD=nrf52840-mdk-usb-dongle SD=s140 NRFUTIL_PORT=/dev/ttyACM0 sd
+
 ## Bluetooth LE REPL
 
 The port also implements a BLE REPL driver. This feature is disabled by default, as it will deactivate the UART REPL when activated. As some of the nRF devices only have one UART, using the BLE REPL free's the UART instance such that it can be used as a general UART peripheral not bound to REPL.
diff --git a/ports/nrf/boards/nrf52840-mdk-usb-dongle/README.md b/ports/nrf/boards/nrf52840-mdk-usb-dongle/README.md
index c39a800d7..f993c7dc5 100644
--- a/ports/nrf/boards/nrf52840-mdk-usb-dongle/README.md
+++ b/ports/nrf/boards/nrf52840-mdk-usb-dongle/README.md
@@ -39,11 +39,10 @@ Follow the standard [nRF Port build instructions](../../README.md); but use
     make BOARD=nrf52840-mdk-usb-dongle
 
 The build artifacts will be created in `build-nrf52840-mdk-usb-dongle`. Once
-built, the easiest way to deploy to the device is to open `firmware.hex` using
+built, the target can be deployed to the device as described in
+[nRFUtil targets](../../README.md#nrfutil-targets).
+
+An alternative way to deploy to the device, is to open `firmware.hex` using
 *nRF Connect* and select *Write*. Detailed instructions can be found on the
 [developer
 wiki](https://wiki.makerdiary.com/nrf52840-mdk-usb-dongle/programming/). 
-
-**Note** that the regular method of deployment for the MicroPython nRF port
-(using `make deploy`) will *not* operate correctly and will overwrite the
-bootloader.
diff --git a/ports/nrf/boards/nrf52840-mdk-usb-dongle/mpconfigboard.mk b/ports/nrf/boards/nrf52840-mdk-usb-dongle/mpconfigboard.mk
index f98d5c88a..ca437418d 100644
--- a/ports/nrf/boards/nrf52840-mdk-usb-dongle/mpconfigboard.mk
+++ b/ports/nrf/boards/nrf52840-mdk-usb-dongle/mpconfigboard.mk
@@ -2,6 +2,18 @@ MCU_SERIES = m4
 MCU_VARIANT = nrf52
 MCU_SUB_VARIANT = nrf52840
 SOFTDEV_VERSION = 6.1.1
-LD_FILES += boards/nrf52840-mdk-usb-dongle/nrf52840_open_bootloader.ld boards/nrf52840_1M_256k.ld
+
+DFU ?= 1
+
+ifeq ($(DFU),1)
+BOOTLOADER=open_bootloader
+BOOTLOADER_VERSION_MAJOR=1
+BOOTLOADER_VERSION_MINOR=2
+FLASHER=nrfutil
+else
+FLASHER=segger
+endif
+
+LD_FILES += boards/nrf52840_1M_256k.ld
 
 NRF_DEFINES += -DNRF52840_XXAA
diff --git a/ports/nrf/boards/nrf52840-mdk-usb-dongle/nrf52840_open_bootloader.ld b/ports/nrf/boards/nrf52840-mdk-usb-dongle/nrf52840_open_bootloader.ld
deleted file mode 100644
index d6c6e743a..000000000
--- a/ports/nrf/boards/nrf52840-mdk-usb-dongle/nrf52840_open_bootloader.ld
+++ /dev/null
@@ -1,2 +0,0 @@
-_ram_start   = 0x20000008;
-_flash_start = 0x1000;
diff --git a/ports/nrf/boards/pca10059/mpconfigboard.mk b/ports/nrf/boards/pca10059/mpconfigboard.mk
index ca555d393..ca437418d 100644
--- a/ports/nrf/boards/pca10059/mpconfigboard.mk
+++ b/ports/nrf/boards/pca10059/mpconfigboard.mk
@@ -2,6 +2,18 @@ MCU_SERIES = m4
 MCU_VARIANT = nrf52
 MCU_SUB_VARIANT = nrf52840
 SOFTDEV_VERSION = 6.1.1
+
+DFU ?= 1
+
+ifeq ($(DFU),1)
+BOOTLOADER=open_bootloader
+BOOTLOADER_VERSION_MAJOR=1
+BOOTLOADER_VERSION_MINOR=2
+FLASHER=nrfutil
+else
+FLASHER=segger
+endif
+
 LD_FILES += boards/nrf52840_1M_256k.ld
 
 NRF_DEFINES += -DNRF52840_XXAA