mac-update: Clean it up so we don't have to remount /system

Bring it more in line with how the sonys do it...  After all our
approach is based on Alin's original approach for the Sonys

It will get called from init.qcom.rc - which is currently in the
device-specific tree

Change-Id: Id3b5b4458303187a908f6bbd54bb99b4f4d141bf