The NIX_PATH assembly now walks over all keys in the given channel
repositories and assembles a NIX_PATH entry from it. Previously it made
assumptions about a set of hardcoded keys being available, which wasn't
ideal as it didn't allow device-entries.
Previously all devices would have NIX_PATH set on the common verion file
defindd in *nix/variables/versions.nix*. Now NIX_PATH is populated with
the versions of the respective device.
