stm32/boards/PYBD_SF2: Restart qspi memory-mapped mode during startup.

The PYBD boards use an F7xx which has an errata 2.4.3:

    Memory-mapped read operations may fail when timeout counter is enabled

This is unfortunate because it means that once QSPI memory-mapped flash is
accessed the QSPI peripheral will leave the CS pin active (low) forever,
which increases power consumption of the SPI flash chip (because it's
active and waiting for commands).  The exact amount of power increase
depends on the flash, but the PYBD_SFx increase by about 2.5mA.

Previously this increase in power only happened when QSPI flash was needed,
eg on PYBD_SF2 when mbedtls or nimble libraries were used.  On PYBD_SF6
it's actually never used.

But with the introduction of ROMFS which lives in the QSPI flash, the
memory is always access on start up to see if the ROMFS contains a valid
image (it must read the memory to find out).  That means these boards
always consume about 2.5mA more after starting up (compared to when ROMFS
is disabled).

The fix in this commit is to explicitly restart the QSPI memory mapped mode
during the start up process.  More precisely, the restart is done after
querying the ROMFS and just before trying to execute `boot.py`.  That's the
right location to keep power consumption permanently down if the QSPI is
never used (eg ROMFS image doesn't exist).

Signed-off-by: Damien George <damien@micropython.org>

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