lib/cbfs: Add fallback to RO region to cbfs_boot_locate

With this change cbfs_boot_locate will check the RO (COREBOOT) region if
a file can not be found in the active RW region. By doing so it is not
required to duplicate static files that are not intended to be updated
to the RW regions.

The coreboot image can still be updated by adding the file to the RW
region.

This change is intended to support VBOOT on systems with a small flash
device.

BUG=N/A
TEST=tested on facebook fbg1701

Change-Id: I81ceaf927280cef9a3f09621c796c451e9115211
Signed-off-by: Wim Vervoorn <wvervoorn@eltan.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36545
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/security/vboot/Kconfig

@@ -220,6 +220,15 @@ config RO_REGION_ONLY
 	  Add a space delimited list of filenames that should only be in the
 	  RO section.
 
+
+config VBOOT_ENABLE_CBFS_FALLBACK
+	bool
+	default n
+	depends on VBOOT_SLOTS_RW_A
+	help
+	  When this option is enabled cbfs_boot_locate will look for a file in the RO
+	  (COREBOOT) region if it isn't available in the active RW region.
+
 menu "GBB configuration"
 
 config GBB_HWID