plymouth: Add extra config field

Signed-off-by: Mathieu A.-Tetreault <alexandretm@amotus.ca>

nixos/modules/system/boot/plymouth.nix

@@ -25,6 +25,7 @@ let
     [Daemon]
     ShowDelay=0
     Theme=${cfg.theme}
+    ${cfg.extraConfig}
   '';
 
 in
@@ -65,6 +66,15 @@ in
         '';
       };
 
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Literal string to append to <literal>configFile</literal>
+          and the config file generated by the plymouth module.
+        '';
+      };
+
     };
 
   };