update_verifier: Allow searching /dev/block.

update_verifier calls bootcontrol HAL to mark the currently booting slot
as successfully booted.

avc: denied { search } for name="block" dev="tmpfs" ino=15510 scontext=u:r:update_verifier:s0 tcontext=u:object_r:block_device:s0 tclass=dir permissive=0
avc: denied { search } for name="block" dev="tmpfs" ino=15510 scontext=u:r:update_verifier:s0 tcontext=u:object_r:block_device:s0 tclass=dir permissive=0

Bug: 29569601
Test: Marlin boots up with no update_verifier denials and 'bootctl is-slot-marked-successful 0' returns 0.
Change-Id: I1baa7819bc829e3c4b83d7168008a5b06b01cc9f