android_ab: Try backup booloader_message

Some devices keep 2 copies of the bootloader_message in the misc
partition and write each in sequence when updating. This ensures that
there is always one valid copy of the bootloader_message. Teach u-boot
to optionally try a backup bootloader_message from a specified offset if
the primary one fails its CRC check.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
diff --git a/common/Kconfig b/common/Kconfig
index 42baca2..f5ad63c 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -936,6 +936,15 @@
 	  allows a bootloader to try a new version of the system but roll back
 	  to previous version if the new one didn't boot all the way.
 
+config ANDROID_AB_BACKUP_OFFSET
+	hex "Offset of backup bootloader control"
+	depends on ANDROID_AB
+	default 0x0
+	help
+	  If non-zero, a backup bootloader message starting at this offset in
+	  the partition will tried in the event that the primary one (starting
+	  at offset 0) fails its checksum.
+
 endmenu
 
 menu "Blob list"