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

"out" can be $OUT_DIR

Bug: 7183057
Change-Id: I7b52d40de4398f481e47f6d6d719fb550d874f73
diff --git a/core/definitions.mk b/core/definitions.mk
index 7834baa..bf9901e 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -147,7 +147,7 @@
 # $(1): directory to search under
 # Ignores $(1)/Android.mk
 define first-makefiles-under
-$(shell build/tools/findleaves.py --prune=out --prune=.repo --prune=.git \
+$(shell build/tools/findleaves.py --prune=$(OUT_DIR) --prune=.repo --prune=.git \
         --mindepth=2 $(1) Android.mk)
 endef