nixos/top-level.nix: Make extensible

nixos/modules/system/activation/top-level.nix

@@ -88,6 +88,8 @@ let
 
       echo -n "${toString config.system.extraDependencies}" > $out/extra-dependencies
 
+      ${config.system.systemBuilderCommands}
+
       ${config.system.extraSystemBuilderCmds}
     '';
 
@@ -96,7 +98,7 @@ let
   # kernel, systemd units, init scripts, etc.) as well as a script
   # `switch-to-configuration' that activates the configuration and
   # makes it bootable.
-  baseSystem = pkgs.stdenvNoCC.mkDerivation {
+  baseSystem = pkgs.stdenvNoCC.mkDerivation ({
     name = "nixos-system-${config.system.name}-${config.system.nixos.label}";
     preferLocalBuild = true;
     allowSubstitutes = false;
@@ -120,7 +122,7 @@ let
 
     # Needed by switch-to-configuration.
     perl = pkgs.perl.withPackages (p: with p; [ FileSlurp NetDBus XMLParser XMLTwig ]);
-  };
+  } // config.system.systemBuilderAttrs);
 
   # Handle assertions and warnings
 
@@ -228,6 +230,24 @@ in
       '';
     };
 
+    system.systemBuilderCommands = mkOption {
+      type = types.lines;
+      internal = true;
+      default = "";
+      description = ''
+        This code will be added to the builder creating the system store path.
+      '';
+    };
+
+    system.systemBuilderAttrs = mkOption {
+      type = types.attrsOf types.unspecified;
+      internal = true;
+      default = {};
+      description = ''
+        Derivation attributes that will be passed to the top level system builder.
+      '';
+    };
+
     system.extraSystemBuilderCmds = mkOption {
       type = types.lines;
       internal = true;