Restore build-art minimal build

Change-Id: I0f4f2e8c6d2505a339cb9a1175d30ed9b229c45d
diff --git a/build/Android.executable.mk b/build/Android.executable.mk
index fe540c0..5cf15be 100644
--- a/build/Android.executable.mk
+++ b/build/Android.executable.mk
@@ -16,8 +16,8 @@
 
 include art/build/Android.common.mk
 
-ART_HOST_EXECUTABLES :=
-ART_TARGET_EXECUTABLES :=
+ART_HOST_EXECUTABLES ?=
+ART_TARGET_EXECUTABLES ?=
 
 ART_EXECUTABLES_CFLAGS :=
 ifeq ($(ART_USE_PORTABLE_COMPILER),true)