nixos/grub: Add support for timeoutStyle

nixos/modules/system/boot/loader/grub/grub.nix

@@ -63,7 +63,9 @@ let
         extraGrubInstallArgs
         extraEntriesBeforeNixOS extraPrepareConfig configurationLimit copyKernels
         default fsIdentifier efiSupport efiInstallAsRemovable gfxmodeEfi gfxmodeBios gfxpayloadEfi gfxpayloadBios
-        users;
+        users
+        timeoutStyle
+      ;
       path = with pkgs; makeBinPath (
         [ coreutils gnused gnugrep findutils diffutils btrfs-progs util-linux mdadm ]
         ++ optional cfg.efiSupport efibootmgr
@@ -425,6 +427,28 @@ in
         '';
       };
 
+      timeoutStyle = mkOption {
+        default = "menu";
+        type = types.enum [ "menu" "countdown" "hidden" ];
+        description = lib.mdDoc ''
+           - `menu` shows the menu.
+           - `countdown` uses a text-mode countdown.
+           - `hidden` hides GRUB entirely.
+
+          When using a theme, the default value (`menu`) is appropriate for the graphical countdown.
+
+          When attempting to do flicker-free boot, `hidden` should be used.
+
+          See the [GRUB documentation section about `timeout_style`](https://www.gnu.org/software/grub/manual/grub/html_node/timeout.html).
+
+          ::: {.note}
+          If this option is set to ‘countdown’ or ‘hidden’ [...] and ESC or F4 are pressed, or SHIFT is held down during that time, it will display the menu and wait for input.
+          :::
+
+          From: [Simple configuration handling page, under GRUB_TIMEOUT_STYLE](https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html).
+        '';
+      };
+
       entryOptions = mkOption {
         default = "--class nixos --unrestricted";
         type = types.nullOr types.str;

nixos/modules/system/boot/loader/grub/install-grub.pl

@@ -75,6 +75,7 @@ my $backgroundColor = get("backgroundColor");
 my $configurationLimit = int(get("configurationLimit"));
 my $copyKernels = get("copyKernels") eq "true";
 my $timeout = int(get("timeout"));
+my $timeoutStyle = get("timeoutStyle");
 my $defaultEntry = get("default");
 my $fsIdentifier = get("fsIdentifier");
 my $grubEfi = get("grubEfi");
@@ -319,6 +320,7 @@ $conf .= "
       set default=$defaultEntryText
       set timeout=$timeout
     fi
+    set timeout_style=$timeoutStyle
 
     function savedefault {
         if [ -z \"\${boot_once}\"]; then