uart-bridge: bump clock to 250 MHz

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

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 <noltari@gmail.com>

Update README with multiple UARTs support

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

uart-bridge: use DEF_BIT_RATE for CDC LC

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

Import project files

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>