add support for building flashable kernel zips

choose device from lunch menu then enter make bootzip, builds flashable boot.img with updated source ramdisk

Change-Id: I07820d5e5e48d5b973901ed2bb3f330744c6498c

Conflicts:
	core/Makefile

Conflicts:
	core/Makefile
	tools/releasetools/boot_flash_from_image