partutil: Refactor MountEFIPartition output

To allow reading other files on the EFI partition, output the EFI
partition mountpoint instead of just the grub config. A constant for the
grub config path is additionally exported.

BUG=b/465210631
TEST=unit tests

Change-Id: Ib00a696e20de9e777be13e8c60a3c761b962fd9c
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/130122
Reviewed-by: Kevin Berry <kpberry@google.com>
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>