add fit bootloader

this is for the belkin rt3200, whose uboot doesn't do
extlinux but can load a fit from a ubifs. It adds the
a kernel+dtb as /boot/fit

modules/outputs.nix

@@ -14,6 +14,7 @@ in
     ./outputs/squashfs.nix
     ./outputs/vmroot.nix
     ./outputs/boot-extlinux.nix
+    ./outputs/boot-fit.nix
     ./outputs/uimage.nix
     ./outputs/updater
   ];

modules/outputs/boot-fit.nix

@@ -0,0 +1,27 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkIf mkEnableOption mkOption types concatStringsSep;
+  inherit (pkgs.pseudofile) dir symlink;
+  cfg = config.boot.loader.fit;
+  o = config.system.outputs;
+  cmdline = concatStringsSep " " config.boot.commandLine;
+  wantsDtb = config.hardware.dts ? src && config.hardware.dts.src != null;
+in {
+  options.boot.loader.fit.enable = mkEnableOption "FIT in /boot";
+
+  config = mkIf cfg.enable {
+    system.outputs.bootfiles = pkgs.runCommand "boot-fit" {} ''
+      mkdir $out
+      cd $out
+      cp ${o.uimage} fit
+    '';
+    filesystem = dir {
+      boot = symlink config.system.outputs.bootfiles;
+    };
+  };
+}