device-lenovo-21bx: New device

(@craftyguy has taken over finishing this patch, so comments below are his)

I think this is ready for review now. With some few exceptions (modem, camera, touchscreen) and some light testing, almost everything works in this initial port. The exceptions are either things I don't have (touchscreen), know they aren't working in the kernel yet (camera?), or are untested since I don't have all the required stuff for it (spare SIM for testing modem).

I've been booting images on a USB drive using the latest EFI fw for the X13s (v1.58).