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.
