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
Merge branch 'cm-10.1' of github.com:CyanogenMod/android_bootable_recovery into cm-10.1

Conflicts:
	roots.c

Change-Id: Ied0390c5ef2cfc1805fb3e6e6bf359fd204976d7