app: aboot: Add check for blank image_header

Device should remain in fastboot mode if a blank image_header is
detected & should not go to recovery by returning -ECORRUPTED_IMAGE.
image_header buffer will read NULL from emmc and all-bits-set from flash
when the boot partition is erased or is empty.

Change-Id: Ib0dc2d7525eafcc359dd9996c3523b6357982b77