Populate BRILLO_PRODUCT_ID from the product config

Using cfgtree.mk, add support to load the brillo
product id directly from a file in the product tree
when configured.

BUG=25343470

(cherry picked from commit 00302c248da7a0200d52efbc9754f0b1c67f466a)

Change-Id: I7e719ea79e4fd8f034c6c8051acf184c313d1a3b
1 file changed