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
Change-Id: I9d54c562a2e623b23b7c1dea9cdbe303786c95dd
diff --git a/Android.mk b/Android.mk
index a518d2f..25796a0 100644
--- a/Android.mk
+++ b/Android.mk
@@ -558,3 +558,10 @@
 TEST_ART_TARGET_SYNC_DEPS :=
 
 include $(art_path)/runtime/openjdkjvm/Android.mk
+
+# Helper target that depends on boot image creation.
+#
+# Can be used, for example, to dump initialization failures:
+#   m art-boot-image ART_BOOT_IMAGE_EXTRA_ARGS=--dump-init-failures=fails.txt
+.PHONY: art-boot-image
+art-boot-image: $(DEFAULT_DEX_PREOPT_BUILT_IMAGE_FILENAME)