Add BOARD_DONT_BUILD_USERDATA_IMG

So board config can choose to not build userdata.img.

Change-Id: I1e6024abb7806492468801e49075fa24c05d2dd1
diff --git a/core/Makefile b/core/Makefile
index 3828f53..869379c 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -899,6 +899,7 @@
     $(filter $(TARGET_OUT_DATA)/%,$(tests_MODULES))
 endif
 
+ifneq (true,$(BOARD_DONT_BUILD_USERDATA_IMG))
 userdataimage_intermediates := \
     $(call intermediates-dir-for,PACKAGING,userdata)
 BUILT_USERDATAIMAGE_TARGET := $(PRODUCT_OUT)/userdata.img
@@ -924,6 +925,8 @@
 userdataimage-nodeps: | $(INTERNAL_USERIMAGES_DEPS)
 	$(build-userdataimage-target)
 
+endif # no BOARD_DONT_BUILD_USERDATA_IMG
+
 #######
 ## data partition tarball
 define build-userdatatarball-target