Refactor unmount_image with unmount_dir

To replace the similiar umount logic in unmout image.

Change-Id: Ia2105a48bbed00ba2571b442e216fccfbcc17f01

lib/img-functions

@@ -22,11 +22,7 @@ function unmount_image () {
 
     # unmount from the chroot
     # Don't use TMP_MOUNT_PATH here, it might not have been set.
-    local MOUNTS
-    MOUNTS=$(grep < /proc/mounts " $TMP_BUILD_DIR/mnt" | awk '{ print $2 }' | sort -r)
-    for M in $MOUNTS; do
-        sudo umount -fl $M || true
-    done
+    unmount_dir "$TMP_BUILD_DIR/mnt"
     if [ -n "$EXTRA_UNMOUNT" ]; then
         $EXTRA_UNMOUNT
     fi