Generalize config for internal hashdist builds

This is needed to build Proteus correctly from within a hashdist build.
The Garnet build is semi-broken at this point in master, and the recommended procedure is to still copy the config.py.garnet.gnu file manually over.  I think my plan for now is just to implement a simple registry for machine overrides that gets activated when a Garnet fen is detected.  I don't think it's high priority - let me know if it is.