Implement the Spinel+HDLCLite protocol

[puddly]

Depends on a few patches to OpenThread to work:

• https://github.com/puddly/openthread/tree/puddly/reboot-bootloader
• https://github.com/puddly/ot-efr32/tree/puddly/bootloader-reboot

One notable problem is that OpenThread doesn't have version numbers:

Detected ApplicationType.SPINEL, version OPENTHREAD/6ff1ac0-dirty; EFR32; Dec 23 2022 18:08:00

One possible solution would be to transform the version into something more parsable, like:

2022.12.23-180800-OPENTHREAD/6ff1ac0-dirty

Alternatively, we can just ignore all version number parsing and always flash firmware if the version number differs.