explicitly check the superblock for differences

When generating incrementals for the system and vendor partitions,
check the first block (which contains the superblock) of the partition
to see if it's what we expect.  If this check fails, give an explicit
log message about the partition having been remounted R/W (the most
likely explanation) and the need to flash to get OTAs working again.

Bug: 17393999
Change-Id: Ifd2132b428dbc4907527291712690204a3664ac0