recovery: fix mounting voldmanaged volumes

Starting from 4.4, voldmanaged volumes now have the mount point set
to "auto", which recovery will have to handle.

This is done by setting the mount point to /storage/[label], and
modifying vold to mount to /storage/[label] instead of
/mnt/media_rw/[label].

Other explored, but dropped, options include:
 - Setting the mount point directly to /mnt/media_rw/[label] does not
   work because vold expects the volume to be in form of /storage/[label]
 - We can also also use a symlink to /mnt/media_rw/[label], but that
   requires mangling with mtab when checking which partitions are mounted
   and also breaks install scripts which make certain assumptions
   (particularly, aries)

Requires http://review.cyanogenmod.org/53733

Change-Id: I4d35fd81edbc9cbed1812caa5c93e5fb5384154a
Signed-off-by: Chirayu Desai <cdesai@cyanogenmod.org>