Fix hanging mount points issues

Mainly fixes the building of target ubuntu image.
Just a couple of minor things like:
1) unreachable lazy umount due to `echo` return code
2) non removing of dissociated loop device
3) forgotten mirror unmounting

Change-Id: Ib991ebe31aa4b7a9f421a38d1dfee3d886177906
Closes-Bug: #1391896
Merge "Fix hanging mount points issues"