nixos/modules: Remove all usages of types.string

And replace them with a more appropriate type

Also fix up some minor module problems along the way

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

@@ -10,7 +10,7 @@ let
   builderUboot = import ./uboot-builder.nix { inherit pkgs configTxt; inherit (cfg) version; };
   builderGeneric = import ./raspberrypi-builder.nix { inherit pkgs configTxt; };
 
-  builder = 
+  builder =
     if cfg.uboot.enable then
       "${builderUboot} -g ${toString cfg.uboot.configurationLimit} -t ${timeoutStr} -c"
     else
@@ -86,7 +86,7 @@ in
 
       firmwareConfig = mkOption {
         default = null;
-        type = types.nullOr types.string;
+        type = types.nullOr types.lines;
         description = ''
           Extra options that will be appended to <literal>/boot/config.txt</literal> file.
           For possible values, see: https://www.raspberrypi.org/documentation/configuration/config-txt/