brillo_config: pull PRODUCT_VERSION from cfgtree

Add support to read the product version from
  config/brillo/product_version

(cherry picked from commit f01bc2175998a3d13e47dc6afe913ae8f036a89e)
BUG=25516395

Change-Id: Id1277112ce7f7c89c9d984ff372bc43007d41016
1 file changed