robert-hh 49e17c8bb0 mimxrt/machine_uart: Implement uart.flush() and uart.txdone().
uart.flush()

flush() will wait until all characters have been sent.To avoid a
permanent lock, a timeout applies depending on the size of txbuf
and the baud rate.

ret = uart.txdone()

ret is True if no transfer is in progress.
ret is False otherwise.
2022-08-31 00:18:21 +10:00
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