Define the Bootloader Control A/B structure

Define the A/B structure for implementation using the misc partition
as storage.  The bootloader_control structure is designed to be put in
the bootloader_message slot_suffix field.

Change-Id: I158bdf3e69b3f327a1dde4c6eb1f907dbaf8939c
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>