updated variables and added defconfigs

scripts/build_image.sh

@@ -23,7 +23,7 @@ case $1 in
             download_sources
         fi
         rootfs
-        if [ "$image_usable_enable" = "true" ]; then
+        if [ "$IMAGE_USABLE_ENABLE" = "true" ]; then
             sdroot_usable
         fi
         sdroot