Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/roles/base/default.nix

@@ -0,0 +1,50 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  imports = [ ];
+
+  options.me = {
+    base.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install base.";
+    };
+  };
+
+  config = lib.mkIf config.me.base.enable {
+    environment.systemPackages = with pkgs; [
+      wget
+      mg
+      rsync
+      libinput
+      htop
+      tmux
+      file
+      usbutils # for lsusb
+      pciutils # for lspci
+      ripgrep
+      strace
+      # ltrace # Disabled because it uses more than 48GB of /tmp space during test phase.
+      trace-cmd # ftrace
+      tcpdump
+      git-crypt
+      gnumake
+      ncdu
+      nix-tree
+      libarchive # bsdtar
+      lsof
+      doas-sudo-shim # To support --sudo for remote builds
+      dmidecode # Read SMBIOS information.
+      ipcalc
+      gptfdisk # for cgdisk
+      nix-output-monitor # For better view into nixos-rebuild
+      nix-serve-ng # Serve nix store over http
+    ];
+  };
+}