example.nix

{ pkgs, lib, config, ... }:

with lib;
let
  cfg = config.stuff;
in {
  options.stuff = {
    chromium = mkEnableOption "chromium";
    firefox = mkEnableOption "firefox";
    flash = mkEnableOption "flash";
  };
  config = {
    environment.systemPackages = (optional cfg.chromium pkgs.chromium) ++ (optional cfg.firefox pkgs.firefox);
    nixpkgs.config = {
      chromium = {
        enablePepperFlash = cfg.flash;
      };
      firefox = {
        enableAdobeFlash = cfg.flash;
      };
    };
  };
}