Test of OpenThread RCP on Silicon Labs hardware

[Nerivec]

Current firmware: https://github.com/Nerivec/silabs-firmware-builder/releases

Current reported state:

• Hardware unstable, see below.

Known issues:

• Hardware MAC ACKing does not appear to set frame pending bit correctly, which some devices do not like (e.g. Innr).
• Random chip crashes after running for a while
  • have to unplug/replug the adapter to be able to start again
  • happens with both USART and EUSART, various baudrates...
  • can't seem to identify any pattern to it, be it device, frame, time...

Known limitations:

• UNIMPLEMENTED: write MAC_SCAN_STATE with SCAN_STATE_BEACON