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.
It can be enabled/disabled by a configuration switch. The code size
increase is 308 bytes, but it requires RAM space for buffers, the larger
UART object and root pointers.
All board pins that have UART's assigned can be used. Baud rate range is
75 Baud to ~2 MBaud.
No flow control yet, and only RX is buffered. TX buffer and flow control
may be added later for SAMD51 with its larger RAM and Flash.
