ART: Add Makefile target for boot image

Add a phony Makefile target that depends on the dexopted boot image.
This allows to test out changes easily, for example, for compile-time
initialization:

 m art-boot-image ART_BOOT_IMAGE_EXTRA_ARGS=--dump-init-failures=fails.txt

Bug: 27265238

(cherry picked from commit fad48e74)

Change-Id: I9d54c562a2e623b23b7c1dea9cdbe303786c95dd