Populate BRILLO_CRASH_SERVER from the product config

Using cfgtree.mk, this change adds support to load the
crash server URL directly from a file in the product tree
during the build.

BUG=25343470
TEST=build image, check in etc for the populated osrelease.d file

(cherry picked from commit e680f358)

Change-Id: I28879ec7b82f3cda2a02a97a116368ca2fbbdc8b