Preserve symlinked OUT_DIR by only clearing contents

Change-Id: I824ecd46d7fd31885024ff20cf5cbd52e2cbb713
diff --git a/core/main.mk b/core/main.mk
index 992f6e1..1048c9b 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1033,7 +1033,7 @@
 
 .PHONY: clean
 clean:
-	@rm -rf $(OUT_DIR)
+	@rm -rf $(OUT_DIR)/*
 	@echo "Entire build directory removed."
 
 .PHONY: clobber