This supports u-boot to check rollback index from
'MEDIATEK_IOT_ROLLBACK_INDEX0'. If the rollback index is invalid,
it will not be able to boot and will change the current boot slot.

Change-Id: I0f7ff2c5e0298e9bbe350584e8aaf08f5eb2b12d
Signed-off-by: Howard Lin <howard-yh.lin@mediatek.com>