core: don't hardcode "out" during findleaves.py invocations.

"out" can be $OUT_DIR

Bug: 7183057
Change-Id: I7b52d40de4398f481e47f6d6d719fb550d874f73
diff --git a/core/main.mk b/core/main.mk
index 25ec74e..47847bf 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -479,7 +479,7 @@
 # Can't use first-makefiles-under here because
 # --mindepth=2 makes the prunes not work.
 subdir_makefiles := \
-	$(shell build/tools/findleaves.py --prune=out --prune=.repo --prune=.git $(subdirs) Android.mk)
+	$(shell build/tools/findleaves.py --prune=$(OUT_DIR) --prune=.repo --prune=.git $(subdirs) Android.mk)
 
 include $(subdir_makefiles)