slot_metadata: add verity_corrupted field

In rootfs context, the verity mode must be supplied by the bootloader
to the kernel.  This patch creates a new verity_corrupted field in the
slot metadata structure to remind that this slot is corrupted from a
dm-verity point of view.

If the bootloader receives the "dm-verity device corrupted" reboot
target, it should set this bit to 1.  If this bit value is 1, the
bootloader should set the veritymode to "eio".

Change-Id: I9335a39d7d009200318c58c53a3139d542788a9b
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>