uart.flush()
flush() will wait until all characters but the last one have been sent.
It returns while the last character is sent. If needed, the calling
code has to add one character wait time. To avoid a permanent lock,
a timeout applies depending on the size of the FIFO and the baud rate.
ret = uart.txdone()
ret is True if no transfer is in progress. It returns already True when
the last byte of a transfer is sent.
ret is False otherwise.
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