nixpkgs: add support for overlays

default.nix

@@ -6,10 +6,8 @@ let
 
   packages = { config, lib, pkgs, ... }: {
     config = {
-      _module.args.pkgs = import nixpkgs {
-        inherit system;
-        inherit (config.nixpkgs) config;
-      };
+      _module.args.pkgs = import nixpkgs config.nixpkgs;
+      nixpkgs.system = system;
     };
   };
 

modules/nix/nixpkgs.nix

@@ -34,11 +34,16 @@ let
     merge = args: fold (def: mergeConfig def.value) {};
   };
 
+  overlayType = mkOptionType {
+    name = "nixpkgs-overlay";
+    description = "nixpkgs overlay";
+    check = builtins.isFunction;
+    merge = lib.mergeOneOption;
+  };
 in
 
 {
   options = {
-
     nixpkgs.config = mkOption {
       default = {};
       example = literalExample
@@ -63,6 +68,39 @@ in
       '';
     };
 
+    nixpkgs.overlays = mkOption {
+      type = types.listOf overlayType;
+      default = [];
+      example = literalExample ''
+        [ (self: super: {
+            openssh = super.openssh.override {
+              hpnSupport = true;
+              withKerberos = true;
+              kerberos = self.libkrb5;
+            };
+          };
+        ) ]
+      '';
+      description = ''
+        List of overlays to use with the Nix Packages collection.
+        (For details, see the Nixpkgs documentation.)  It allows
+        you to override packages globally. This is a function that
+        takes as an argument the <emphasis>original</emphasis> Nixpkgs.
+        The first argument should be used for finding dependencies, and
+        the second should be used for overriding recipes.
+      '';
+    };
+
+    nixpkgs.system = mkOption {
+      type = types.str;
+      example = "x86_64-darwin";
+      description = ''
+        Specifies the Nix platform type for which NixOS should be built.
+        If unset, it defaults to the platform type of your host system.
+        Specifying this option is useful when doing distributed
+        multi-platform deployment, or when building virtual machines.
+      '';
+    };
   };
 
   config = {