Port AOKP: [1/3] Install persistence support
Copy files from OUT/install to ZIP/install and extract to /tmp/install
at install time.  This is an extensible framework that may be used for
other install tasks as needed.  Note that none of the install files are
only extracted to a ramdisk, they are never written to /system.

Run script persist.sh with argument save prior to formatting /system
and again with argument restore after /system is installed.  This may
be explicitly disabled by setting TARGET_DISABLE_PERSIST := true.