Add a WITH_DEXOPT_BOOT_IMG_ONLY configuration option.

If WITH_DEXOPT_BOOT_IMG_ONLY=true and WITH_DEXPREOPT=true then we will
only preopt the boot.art and boot.oat files, leaving everything else to
be compiled at first boot. This has fast startup times of WITH_DEXPREOPT
but has a smaller space usage and allows one to update the non-image
parts of /system without reflashing.

Bug: 16938924

Change-Id: Ib366b6b5ad80f7078f01bf51f9fbc29ea7e5d777