move squashfs kernel options into module

modules/squashfs.nix

@@ -12,5 +12,9 @@ in
   config = mkIf (config.rootfsType == "squashfs") {
     system.outputs.rootfs =
       liminix.builders.squashfs config.filesystem.contents;
+    kernel.config = {
+      SQUASHFS = "y";
+      SQUASHFS_XZ = "y";
+    };
   };
 }