nix: add plymouth

nix/modules/boot.nix

@@ -3,7 +3,8 @@
 # theming of grub
 
 {
-  boot.loader.grub = {
+  boot = {
+    loader.grub = {
       theme = pkgs.stdenv.mkDerivation {
         pname = "hyperfluent";
         version = "v1.0.1";
@@ -16,4 +17,18 @@
         installPhase = "cp -r nixos $out";
       };
     };
+
+    initrd.systemd.enable = true;
+
+    plymouth = {
+      enable = true;
+      theme = "rings";
+      themePackages = with pkgs; [
+        # By default we would install all themes
+        (adi1090x-plymouth-themes.override {
+          selected_themes = [ "rings" ];
+        })
+      ];
+    };
+  };
 }