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.
2251cb774b