REPL/console via hardware UART

[dhalbert]

Provide support for accessing the REPL over UART. There are some compile switches for this, but I don't know how complete it is on the inside. There was support long ago because of the ESP8266. The BLE-only workflow would use a similar internal path.

One request: #4752 (comment)

Related to #721.

[tannewt]

Yup, I may rethink this with the BLE work. It may involve giving UARTs a lifetime similar to displays.

[fkpwolf]

I also want this feature since virtula/emulated UART is not stable when device usb is plugin and out. I can't debug device if error happened at the time.

[tannewt]

@fkpwolf Please file a separate issue for what is unstable. It shouldn't be.

[tannewt]

We do have this at build time through DEBUG_UART. We don't currently allow boot.py to set it up though.