Add a convenient target for building a zip file of javadocs: $(LOCAL_MODULE)-docs.zip

Change-Id: I97fcc0d467f3b7582570b9be7e15acc0a5b550a5
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index 0d3094d..fd7f338 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -231,6 +231,8 @@
 	@mkdir -p $(dir $@)
 	$(hide) ( F=$$(pwd)/$@ ; cd $(PRIVATE_DOCS_DIR) && zip -rq $$F * )
 
+$(LOCAL_MODULE)-docs.zip : $(out_zip)
+
 $(call dist-for-goals,docs,$(out_zip))
 
 endif