Build: Add ART_BOOT_IMAGE_EXTRA_ARGS

Allow to modify the boot image compilation. Can be used to easily
dump initialization failures.

Bug: 27265238
Change-Id: I9d54c562a2e623b23b7c1dea9cdbe303786c95dd
diff --git a/core/dex_preopt_libart_boot.mk b/core/dex_preopt_libart_boot.mk
index 1c8fb40..69102d1 100644
--- a/core/dex_preopt_libart_boot.mk
+++ b/core/dex_preopt_libart_boot.mk
@@ -62,4 +62,4 @@
 		--instruction-set-features=$($(PRIVATE_2ND_ARCH_VAR_PREFIX)DEX2OAT_TARGET_INSTRUCTION_SET_FEATURES) \
 		--android-root=$(PRODUCT_OUT)/system --include-patch-information --runtime-arg -Xnorelocate --no-generate-debug-info \
 		--multi-image --no-inline-from=core-oj.jar \
-		$(PRODUCT_DEX_PREOPT_BOOT_FLAGS) $(GLOBAL_DEXPREOPT_FLAGS) $(COMPILED_CLASSES_FLAGS)
+		$(PRODUCT_DEX_PREOPT_BOOT_FLAGS) $(GLOBAL_DEXPREOPT_FLAGS) $(COMPILED_CLASSES_FLAGS) $(ART_BOOT_IMAGE_EXTRA_ARGS)