Fixed device or resource busy issue in EXIT trap

cleanup for ramdisk failed to umount TMP_BUILD_DIR with device or
resource busy error.

The patch unmounts all the mountpoints under TMP_BUILD_DIR and
detaches loop devices associated with TMP_IMAGE_PATH.

The unmounts are applied with both force(-f) and lazy(-l)
options. Force option is only for NFS mounts, it's kept here since no
harm for lazy option.

Change-Id: I84035e6a003d8135186b2fda3facbd2c37967529
Refactor unmount_image with unmount_dir

To replace the similiar umount logic in unmout image.

Change-Id: Ia2105a48bbed00ba2571b442e216fccfbcc17f01
Merge "Fixed device or resource busy issue in EXIT trap"
Merge "Refactor unmount_image with unmount_dir"