base: fix external sdcard mountpoint

It needs to add that line on init.xxxx.rc on rootdir folders.

"chown root root /mnt/media_rw"

Needed for devices that has some troubles mounting storages.

Credits: http://forum.xda-developers.com/showthread.php?t=2537793

Change-Id: I0c9e2ae5439c8d8a762904e2760eb6590ce77a62