Normalize the product makefile path in inherit-product.

Some vendor product makefiles call $(inherit-product) on the same
product makefile with different paths, by using "../" in relative paths.
However inherit-product requires unique path as ID for a product
makefile, for bookkeeping purpose.
Normalize the product makefile path in inherit-product, if the path
contains "../".

Change-Id: I1a864ce120c713d8e79ec179213b9fc9352aba53