recovery: bu: Support verity, incremental OTAs, and encrypted data

 * Do block device backups for partitions that are either:
   - Not mountable due to unknown/unsupported filesystems
   - Declared read-only in fstab
   - Marked as verify (dm-verity)

 * Fallback to block device backups if filesystem mount fails.  This
   will be the case for devices with encrypted data.

 * When mounting filesystems, default to honoring the read-only flag
   in fstab.

JIRA: CYNGNOS-322

Change-Id: I6d2c8ab81b4e2335661fcbbb1cf54bad31a55af3