cos-kernel: Use root disk p12 as EFI partition

If a second boot disk has been mounted, the
/dev/disk/bypartlabel/EFI-SYSTEM link can be overwritten. On COS,
it's more reliable to use partition 12 of the root disk instead,
which should always be the actual EFI partition.

BUG=b/567952125
TEST=presubmit, unlock and deploy kernel on x86 and arm64

Change-Id: If44102e4f85ca14ef1b1a5667024568b43fe0afe
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/219608
Reviewed-by: Derek Taylor <ddtaylor@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: Kevin Berry <kpberry@google.com>