add mechanism to include local (not in git) config

and an example of how you'd use it to add eculocate

configuration.nix

@@ -1,6 +1,9 @@
 { lib, config, pkgs, ... }:
 
 let
+  local_config = if (builtins.pathExists ./local/module.nix)
+                 then ./local/module.nix
+                 else null;
   saturn = pkgs.callPackage ./pkgs/saturn {};
 
   drm-framebuffer =  pkgs.stdenv.mkDerivation {
@@ -26,7 +29,8 @@ let
 in {
   imports = [
     (import ../mobile-nixos/lib/configuration.nix { device = "motorola-harpia"; })
-  ];
+  ] ++ lib.optional (local_config != null) local_config;
+
 
   config = {
     nixpkgs.overlays = [

local-module-eculocate.nix

@@ -0,0 +1,24 @@
+{ lib, config, pkgs, ... }:
+
+# this isn't really part of biscuit, because "eculocate" is a lot more
+# niche than the rest of it. But it serves as an example of the kind of
+# thing you could put in local/module.nix
+
+let
+  eculocate = pkgs.callPackage <eculocate/client> {};
+in {
+  imports = [ <eculocate/client/module.nix> ];
+  config = {
+    programs.eculocate = {
+      enable = true;
+      otaPrivateKeyFile = "/etc/secrets/eculocate/ota.key";
+      blePrivateKeyFile = "/etc/secrets/eculocate/client.key";
+      blePeerKeyFile = "/etc/secrets/eculocate/device.pub";
+      outputPath = "/tmp/eculocate";
+    };
+    services.avahi = {
+      nssmdns4 = true;
+      enable = true;
+    };
+  };
+}