Compilation of Linux and uboot with extra config

I am trying to compile a nixos image. I added some extra configs in my uboot and Linux package. Nix doesn’t recompile them. It takes previous compiled packages as Hashes remains same. I tried to delete output of uboot and Linux but I can not, it says the packages are still alive. How to enable compilation of these packages?

You need to update the package hash when you change a package. How did you make your changes to the source?