Allow disabling DroidDoc build.

Change-Id: Ia842827a46b107bcef0d176deb0ab651e04bfa04
diff --git a/core/config.mk b/core/config.mk
index 57c201a..6cbe39d 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -75,11 +75,16 @@
 BUILD_JAVA_LIBRARY:= $(BUILD_SYSTEM)/java_library.mk
 BUILD_STATIC_JAVA_LIBRARY:= $(BUILD_SYSTEM)/static_java_library.mk
 BUILD_HOST_JAVA_LIBRARY:= $(BUILD_SYSTEM)/host_java_library.mk
-BUILD_DROIDDOC:= $(BUILD_SYSTEM)/droiddoc.mk
 BUILD_COPY_HEADERS := $(BUILD_SYSTEM)/copy_headers.mk
 BUILD_NATIVE_TEST := $(BUILD_SYSTEM)/native_test.mk
 BUILD_HOST_NATIVE_TEST := $(BUILD_SYSTEM)/host_native_test.mk
 
+ifneq ($(DISABLE_DROIDDOC),)
+BUILD_DROIDDOC:= $(BUILD_SYSTEM)/clear_vars.mk
+else
+BUILD_DROIDDOC:= $(BUILD_SYSTEM)/droiddoc.mk
+endif
+
 -include cts/build/config.mk
 
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