Revert "grub: removed orphaned mkOption configurationName"

The configurationName option value is still used by NixOS, this removal
breaks grub users.

This reverts commit bd811d32b4.

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

@@ -86,6 +86,15 @@ in
         '';
       };
 
+      configurationName = mkOption {
+        default = "";
+        example = "Stable 2.6.21";
+        type = types.str;
+        description = ''
+          GRUB entry name instead of default.
+        '';
+      };
+
       extraPrepareConfig = mkOption {
         default = "";
         type = types.lines;