recovery: use just the mount point instead of the full path

When resorting to using mount because a block device uses 'auto'
filesystem, we should use the mount point and not the full path.
This fixes an issue with nandroid to external sd on yuga.

Change-Id: Idbff944ea6d5af9112644c2ed46e23ea0a33e5ee
Merge "recovery: use just the mount point instead of the full path" into cm-10.1