rootdir: Execute update_verifier for A/B update.

update_verifier verifies the updated partitions and marks the current
slot as having booted successfully. It needs to be triggered prior to
the start of the framework, otherwise it won't be able to fall back to
the old system without a data wipe.

Bug: 26039641
Change-Id: I6fd183cdd3dfcc72feff2a896368158875b28591