ANDROID: dm: Add android verity target

ANDROID: dm: Add android verity target

This device-mapper target is virtually a VERITY target. This
target is setup by reading the metadata contents piggybacked
to the actual data blocks in the block device. The signature
of the metadata contents are verified against the key included
in the system keyring. Upon success, the underlying verity
target is setup.

BUG: 27175947

Change-Id: I7e99644a0960ac8279f02c0158ed20999510ea97
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Git-commit: 36d01a590d035804dc100e105facf4049cf6fe43
Git-repo: https://android.googlesource.com/kernel/common
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>