vendor_du : preserve custom dpi between builds
	modified:   config/common.mk
	new file:   prebuilt/bin/99-backup.sh

Change-Id: I26ebadd88dfce2c087f4718b644bb66889af1ef5
Merge "vendor_du : preserve custom dpi between builds 	modified:   config/common.mk 	new file:   prebuilt/bin/99-backup.sh" into du44