build: Add extra build customization for /product

This makes /product more consistent with /device & /vendor

BUG: 22030305

Change-Id: Ic65bfde7fb3e9fab8c8fb8d99af15404574abde0
diff --git a/core/definitions.mk b/core/definitions.mk
index ecc7253..ce7602b 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2547,6 +2547,7 @@
 # Include any vendor specific definitions.mk file
 -include $(TOPDIR)vendor/*/build/core/definitions.mk
 -include $(TOPDIR)device/*/build/core/definitions.mk
+-include $(TOPDIR)product/*/build/core/definitions.mk
 
 # broken:
 #	$(foreach file,$^,$(if $(findstring,.a,$(suffix $file)),-l$(file),$(file)))