make jffs2 module provide o.rootfs, conditionally

it's enabled if config.rootfsType == "jffs2"

modules/squashfs.nix

@@ -0,0 +1,17 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (pkgs) liminix;
+  inherit (lib) mkIf;
+in
+{
+  config = mkIf (config.rootfsType == "squashfs") {
+    outputs = rec {
+      rootfs = liminix.builders.squashfs config.filesystem.contents;
+    };
+  };
+}