build/roms: warn when grub_scan_disk is set incorrectly

resources/scripts/build/boot/roms_helper

@@ -68,6 +68,7 @@ source "resources/coreboot/${board}/board.cfg"
 if [ "${grub_scan_disk}" != "both" ] && \
 	[ "${grub_scan_disk}" != "ata" ] && \
 	[ "${grub_scan_disk}" != "ahci" ]; then
+	printf "build/roms: Target %s defines an invalid grub_scan_disk setting. Defaulting to 'both'.\n" "${board}"
 	grub_scan_disk="both"
 	# erroring out would be silly. just use the default
 fi