Add bootloader_message_ab struct.

Move A/B-specific fields, effectively slot_suffix as in this CL, out of
bootloader_message struct. Those A/B-specific fields are expected to be
managed by the A/B-bootloader or boot control HAL, which shouldn't be
cleared by recovery or uncrypt.

Round up the bootloader_message struct to 2-KiB. Round up the
bootloader_message_ab struct to 4-KiB.

Bug: 29193360
Change-Id: I5740a612456da6d4d3b00e49a202d8f689d32690