C-RoM - Persistence - Add installer persistence [3/3]

Add persist.sh to save and restore data across installs.  Initial data
includes property ro.sf.lcd_density and file etc/hosts.  Behavior is
controlled via configuration file etc/persist.conf.

Change-Id: Ib61926395237ef877777366d8b792fa13677be5e
Signed-off-by: Stefano Mura [STELIX] <ssspinni@gmail.com>