netboot: allow build on aarch64-linux

pkgs/tools/networking/netboot/default.nix

···
   meta = with lib; {
     description = "Mini PXE server";
     maintainers = [ maintainers.raskin ];
-    platforms = ["x86_64-linux"];
+    platforms = [ "x86_64-linux" "aarch64-linux" ];
     license = lib.licenses.free;
   };
 }