modules/outputs/ubifs: expose `rootubifs` rather than `rootfs`

I believe there should be another module exposing `rootubifs` as `rootfs`
or let any other module just subsume that component like `zyxel-nwa-fit` output.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

modules/outputs/ubifs.nix

@@ -33,7 +33,7 @@ in
     };
     boot.initramfs.enable = true;
     system.outputs = {
-      rootfs =
+      rootubifs =
         let
           inherit (pkgs.pkgsBuildBuild) runCommand mtdutils;
           cfg = config.hardware.ubi;