Add USB-serial capabilities

[jeanthom]

STM32 dongles have 3 built-in serial IF, we could use them in addition to JTAG (bluepill targets mainly).

Inspiration : https://github.com/satoshinm/pill_serial

[jeanthom]

We can't both use the three USART and keep the current Bluepill pinout.

Should we use only two out of the three USART or break the existing pinout instead?

[zoobab]

@jeanthom sure break the pinout

[jeanthom]

I'm not sure about breaking compatibility, maybe having a BluePill ALT (breaking compatibility with the current BluePill pinout) could be a solution. Adding a new platform/pinout is fairly easy.

[zoobab]

Maybe reuse the usb-cdc code from BMP:

https://github.com/blacksphere/blackmagic/blob/master/src/platforms/common/cdcacm.c

[zoobab]

Actually the code of pill_serial is from BMP:

https://github.com/satoshinm/pill_serial/blob/master/src/cdcacm.c

[zoobab]

But since we are using unicoremx, the includes might be different...

[jeanthom]

I think I might start by porting the pill_serial code to ucmx than integrate it to DirtyJTAG. This way I'll better understand the code, and avoid writing unnecessary code.

[KnorkFox]

One serial port would be enough, on the blue pill board.
Three available TTL ports add only confusion to the user, which port to use. The dirtyjtag-pico has only one serial port available.