nixos/systemd-boot: add edk2-uefi-shell boot option

We already have a edk2-uefi-shell package in nixpkgs, but adding it to
systemd-boot was somewhat tedious. Now it's a single line of nix.

nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix

@@ -29,6 +29,8 @@ let
           $out
       '';
 
+  edk2ShellEspPath = "efi/edk2-uefi-shell/shell.efi";
+
   systemdBootBuilder = pkgs.substituteAll rec {
     name = "systemd-boot";
 
@@ -72,6 +74,8 @@ let
 
     netbootxyz = optionalString cfg.netbootxyz.enable pkgs.netbootxyz-efi;
 
+    edk2-uefi-shell = optionalString cfg.edk2-uefi-shell.enable pkgs.edk2-uefi-shell;
+
     checkMountpoints = pkgs.writeShellScript "check-mountpoints" ''
       fail() {
         echo "$1 = '$2' is not a mounted partition. Is the path configured correctly?" >&2
@@ -343,6 +347,29 @@ in
       };
     };
 
+    edk2-uefi-shell = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Make the EDK2 UEFI Shell available from the systemd-boot menu.
+          It can be used to manually boot other operating systems or for debugging.
+        '';
+      };
+
+      sortKey = mkOption {
+        type = types.str;
+        default = "o_edk2-uefi-shell";
+        description = ''
+          `systemd-boot` orders the menu entries by their sort keys,
+          so if you want something to appear after all the NixOS entries,
+          it should start with {file}`o` or onwards.
+
+          See also {option}`boot.loader.systemd-boot.sortKey`..
+        '';
+      };
+    };
+
     extraEntries = mkOption {
       type = types.attrsOf types.lines;
       default = { };
@@ -476,6 +503,9 @@ in
       (mkIf cfg.netbootxyz.enable {
         "efi/netbootxyz/netboot.xyz.efi" = "${pkgs.netbootxyz-efi}";
       })
+      (mkIf cfg.edk2-uefi-shell.enable {
+        ${edk2ShellEspPath} = "${pkgs.edk2-uefi-shell}/shell.efi";
+      })
     ];
 
     boot.loader.systemd-boot.extraEntries = mkMerge [
@@ -493,6 +523,13 @@ in
           sort-key ${cfg.netbootxyz.sortKey}
         '';
       })
+      (mkIf cfg.edk2-uefi-shell.enable {
+        "edk2-uefi-shell.conf" = ''
+          title  EDK2 UEFI Shell
+          efi    /${edk2ShellEspPath}
+          sort-key ${cfg.edk2-uefi-shell.sortKey}
+        '';
+      })
     ];
 
     boot.bootspec.extensions."org.nixos.systemd-boot" = {

nixos/tests/systemd-boot.nix

@@ -339,6 +339,21 @@ in
     '';
   };
 
+  edk2-uefi-shell = makeTest {
+    name = "systemd-boot-edk2-uefi-shell";
+    meta.maintainers = with pkgs.lib.maintainers; [ iFreilicht ];
+
+    nodes.machine = { ... }: {
+      imports = [ common ];
+      boot.loader.systemd-boot.edk2-uefi-shell.enable = true;
+    };
+
+    testScript = ''
+      machine.succeed("test -e /boot/loader/entries/edk2-uefi-shell.conf")
+      machine.succeed("test -e /boot/efi/edk2-uefi-shell/shell.efi")
+    '';
+  };
+
   memtestSortKey = makeTest {
     name = "systemd-boot-memtest-sortkey";
     meta.maintainers = with pkgs.lib.maintainers; [ julienmalka ];