Build only userdata.img.

Use build command like:
$make -j16 showcommands userdataimage dist TARGET_PRODUCT=trygon_with_data_preloaded TARGET_BUILD_VARIANT=user

Change-Id: I964490e03090c3696d1595c92363273e35cfaed7
diff --git a/core/main.mk b/core/main.mk
index 4a530d0..10a01ce 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -720,6 +720,10 @@
 .PHONY: userdataimage
 userdataimage: $(INSTALLED_USERDATAIMAGE_TARGET)
 
+ifneq (,$(filter userdataimage, $(MAKECMDGOALS)))
+$(call dist-for-goals, userdataimage, $(BUILT_USERDATAIMAGE_TARGET))
+endif
+
 .PHONY: userdatatarball
 userdatatarball: $(INSTALLED_USERDATATARBALL_TARGET)