TODO: serial console

[robertlipe]

Current workarounds include the tiny display on longan nano and using gdb to p msgbuf on a crash or interrupt, but at time point we'll need to type things at the system and that needs input. Pick one or more options from:

SEGGER's version of SemiHosting
Use the real serial pins. (Abstract it to K210 and GD32V) Writing more tty drivers. Ugh.
For GD32V, I think we could treat it like a CDC(ACM) device so it would show up as a USB serial port. The libraries from GD are quite good, but that's a terrifying quantity of code to be spinning through that could be called pre-boot or during a panic handler,