build: Include pre-pathmap extra config

 * We need to mess with the pathmap for building simplerecovery in user
   builds, but previously there was no way to do this externally.
 * Source an optional config file before loading the pathmap so that we
   can do some magic without having to set environment variables outside
   the normal build system.

Change-Id: I6132f9e6ffa04678378a5a00ba37b98cd4d9f773