[2/3] Install persistence support

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: I946d620e06a8dc63a6abec862a7517f8b458f2d1

Conflicts:
	configs/common.mk
Merge CarbonDev via jb2