configuration.nix

...
nixpkgs.overlays = [
  (import openocd-master-overlay.nix)
];
...

openocd-master-overlay.nix

self: super:
{
  openocd-master = super.openocd.overrideAttrs (attrs: {
    pname = "openocd-unstable";
    version = "2020-05-31";

    src = self.fetchgit {
      url = "https://git.code.sf.net/p/openocd/code";
      rev = "d8ac0086f957fabf63d596a3b8d396b110bc696d";
      sha256 = "0rc3f2q204f1sj75i8fqnqcnkjvagr6bdnxip2dr11ngr0xnbww2";
      fetchSubmodules = true;
    };

    patches = [];

    nativeBuildInputs = attrs.nativeBuildInputs ++ [ self.autoreconfHook ];
  });
}