config: Use standard inherit-product-if-exists for vendor/extra

* prepend-product-if-exists was developed specifically to handle some
  special cases where Cyanogen Inc configuration ordering needed to be
  handled ahead of the standard inclusion order.

Change-Id: Ia3e041fb9e3c84d0763d18e4a401121bc210b976

config/common.mk

@@ -361,4 +361,4 @@ endif
 -include $(WORKSPACE)/build_env/image-auto-bits.mk
 -include vendor/lineage/config/partner_gms.mk
 
-$(call prepend-product-if-exists, vendor/extra/product.mk)
+$(call inherit-product-if-exists, vendor/extra/product.mk)