Add the possiblity of using secondary storage for user 0

Since Android 4.2 it is not possible to use external SD card to use for
application data (such as downloaded content for games). This patch adds
the possibility to use external SD card as a storage for user ID 0.
Requires the path to be defined by SECONDARY_STORAGE and used property
"persist.sys.env_use_sec_storage" to check if it's to be used. The property
is to be set in "Advanced Settings" under device tree (this should be
available to user ID 0 only!)

Change-Id: I3288de270bb4fe03171a8cf9d53db111bf58b8b7