Added mounting and umounting /target

It turned out that in some cases it is necessary to umount
all mount points which are connected to a particular disk
before creating other partitions on that disk.

Change-Id: I43a2de132415d026a6dc096e2b25ce0ab4cce915
Closes-Bug: #1297792
Merge "Added mounting and umounting /target" into stable/4.1