fastboot: /mnt/scratch refined access on userdebug

Already has permissions to remove the scratch partition, but to allow
more refined cleansing (eg: just remove vendor override), need the
ability to mount and scrub overlay directories.

Test: manual
Bug: 117605276
Change-Id: Ibc272c0aa7ce207280023912f5f119ccf5079a7f
2 files changed