Unmount internal mounts on finalise errors

This is only one line, but it takes a lot to untangle ...  basically
the current "correct" path is:

---
 mk_build_dir()
  -> sets trap trap_cleanup EXIT

 ... stuff ..

 mount_proc_dev_sys
  -> mounts $TMP_MOUNT_PATH/<proc,dev.sysfs>

 pre-finalise.d
 finalise.d

 unmount_image $TMP_BUILD_DIR/mnt # nb == $TMP_MOUNT_PATH
  -> unmount_dir()
   -> recursive unmount everything inside TMP_MOUNT_PATH

 TMP_IMAGE_PATH=$(dib-block-device getval image-path)
 export TMP_IMAGE_PATH

 dib-block-device umount
 dib-block-device cleanup

 ... actually cleanup directories ...
---

Our current failure exit trap does:

---
 dib-block-device umount
 unmount_image
 ...
---

Note this is the *opposite* of what is done in the correct exit path.
In the failure case, if a script fails in the finalise stages it leads
to /proc, /sys, /dev etc. still being mounted inside the image; the
"dib-block-device umount" call doesn't know anything about these
mounts and tries to unmount the parent directory, and we get a hard
failure with a busy mount, and all the mounts are subsequently leaked.

Note that "unmount_dir", which is ultimately called by
"unmount_image", already knows to skip those mounts that
"dib-block-device umount" manages (this is the DIB_MOUNTPOINTS list).
This is further evidence it should be called *before* the
dib-block-device umount.

Change-Id: Ibef3ce9d1167b9c4ff3d5717b113cd3ed374f5e3

diskimage_builder/lib/img-functions

@@ -39,8 +39,8 @@ function trap_cleanup() {
 }
 
 function cleanup () {
-    dib-block-device umount
     unmount_image
+    dib-block-device umount
     cleanup_build_dir
     cleanup_image_dir
 }