github: force Ubuntu 20.04

Apparently, ubuntu-latest still points to Ubuntu 18.04, which uses an older
and incompatible version of gcc-arm-none-eabi.

Signed-off-by: Álvaro Fernández Rojas <[email protected]>

Update README with multiple UARTs support

Signed-off-by: Álvaro Fernández Rojas <[email protected]>

uart-bridge: use DEF_BIT_RATE for CDC LC

Signed-off-by: Álvaro Fernández Rojas <[email protected]>

Import project files

Signed-off-by: Álvaro Fernández Rojas <[email protected]>