samd/machine_uart: Implement uart.txdone() and uart.flush().

Using the stream method for uart.flush().

uart.txdone() returns True, if the uart not busy, False otherwise.

uart.flush() waits until all bytes have been transmitted or a timeout
triggers.  The timeout is determined by the buffer size and the baud rate.

Also fix two inconsistencies when not using txbuf:
- Report in ioctl as being writeable if there is room in the tx buffer,
  only if it is configured.
- Print the txbuf size if configured.

0 files changed, 0 insertions, 0 deletions