[Skiboot] Towards skiboot 5.1.2

[Stewart Smith]

Hi all,

My current thoughts is that I want to tag skiboot-5.1.2 this week, and
include both the terminate-immediate work from Vasant and the final bits
of opal-prd enhancements from Neelesh (I've already merged the CAPI fix
from Ian).

This will head into op-build for op-build v1.7, which will form the
basis for several releases, so it's important that it's a fairly solid
release that we're happy to support for a decent amount of time.

cheers,
stewart