Make GSI target to export bootimage to DIST

This is to support a bootimage only target in AB.

Bug: 198363484
Test: lunch aosp_arm64-userdebug; make bootimage dist

Change-Id: I9a32c365f635ec4693675a1969d7a1e684c8f55a
diff --git a/core/main.mk b/core/main.mk
index 8ba1396..de9e2e7 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1596,6 +1596,10 @@
 .PHONY: bootimage
 bootimage: $(INSTALLED_BOOTIMAGE_TARGET)
 
+ifeq (true,$(PRODUCT_EXPORT_BOOT_IMAGE_TO_DIST))
+$(call dist-for-goals, bootimage, $(INSTALLED_BOOTIMAGE_TARGET))
+endif
+
 .PHONY: bootimage_debug
 bootimage_debug: $(INSTALLED_DEBUG_BOOTIMAGE_TARGET)