Make sure disabling ST's flashlayout & partitions work as expected

The current ST's .bbclass files do not really checks if
ENABLE_PARTITIONS_IMAGE and ENABLE_FLASHLAYOUT_CONFIG are unset.
This silently breaks third-party distributions that use
their own partitions layout and root directory structures.

Signed-off-by: Bumsik Kim <k.bumsik@gmail.com>

classes/st-partitions-image.bbclass

@@ -38,6 +38,10 @@ python __anonymous () {
 }
 
 image_rootfs_image_clean_task () {
+    if [ ${ENABLE_PARTITIONS_IMAGE} -ne "1" ]; then
+        return
+    fi
+
     for name in ${PARTITIONS_IMAGE};
     do
         if `echo ${IMAGE_NAME} | grep -q $name` ;