Since uart.write() of the nrf port waits until all bytes but the last
one have been sent, uart.flush() and uart.txdone() are implemented
as empty functions to provide API consistency.
uart.flush()
flush() will always return immediately, even if the last byte
may still be sent.
ret = uart.txdone()
uart.txdone() will always return True, even if the last byte
may still be sent.
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