[Feature requiest]: UART and I2C support for Duet 3 expansion/tool boards

[droftarts]

See https://forum.duet3d.com/topic/36732/uart-on-3hc/

RRF does not currently support UART serial communication on Duet 3 expansion/tool boards. The 3HC, 1HCL and Roto toolboard have IO connections that could be used for UART. Please could support for these be added.

I2C is currently not support on any Duet 3 board, though there are other feature requests for I2C on mainboards:
#692
#693
The 3HC and 1HCL (not sure about the Roto) have IO connections that can also support I2C. If we add I2C support for the mainboards, please add support for expansion/tool boards.

[dc42]

@droftarts please split this into separate feature requests. The I2C implementation on main boards and UART implementation on expansion boards are unrelated features.