Allow OS specific settings

lib/modules/os.nix

@@ -0,0 +1,38 @@
+{ lib, ... }:
+  let
+    inherit (lib)
+      mkOption
+      types
+    ;
+  in {
+    options = {
+      valhalla = mkOption {
+        type = types.submodule (
+          { extendModules, ... }:
+            let
+              osVariant = extendModules { };
+              linuxVariant = osVariant.extendModules { };
+              windowsVariant = osVariant.extendModules { };
+            in {
+              options = {
+                linux = mkOption {
+                  inherit (linuxVariant) type;
+                  description = "The Linux specific configuration.";
+                  default = {};
+                  visible = "shallow";
+                };
+
+                windows = mkOption {
+                  inherit (windowsVariant) type;
+                  description = "The Windows specific configuration.";
+                  default = {};
+                  visible = "shallow";
+                };
+              };
+            });
+
+        description = "Configuration for PortValhalla.";
+        default = {};
+      };
+    };
+  }