Enable dm-verity on shamu

Enable dm-verity for the system partition. Store state in the metadata
partition, and display a warning if dm-verity is started in logging

Increasing read-ahead for dm-verity devices from the default 512 to
2048 improves sequential read speeds 13-18% for block sizes 1k-2k and
~30% for larger block sizes (tested up to 32k), without considerably
affecting 512 byte block reads.

Change-Id: I9422a57b5315e69cf0610262259b13ae9e370fd3
6 files changed