Implement files under /etc

nix/modules/system-manager.nix

@@ -0,0 +1,34 @@
+{ lib
+, pkgs
+, config
+, ...
+}:
+{
+  options.system-manager = {
+    services = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+
+    etcFiles = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+  };
+
+  config = {
+    # Avoid some standard NixOS assertions
+    boot = {
+      loader.grub.enable = false;
+      initrd.enable = false;
+    };
+
+    assertions = lib.flip map config.system-manager.etcFiles (entry:
+      {
+        assertion = lib.hasAttr entry config.environment.etc;
+        message = lib.concatStringsSep " " [
+          "The entry ${entry} that was passed to system-manager.etcFiles"
+          "is not present in environment.etc"
+        ];
+      }
+    );
+  };
+}