system: install busybox with low priority

it is a good fallback for common system commands

system/common/busybox.nix

@@ -0,0 +1,18 @@
+{ config, pkgs, lib, ... }:
+let
+  cfg = config.nixfiles.common.busybox;
+in {
+  options.nixfiles.common.busybox.enable = lib.mkEnableOption "" // {
+    description = ''
+      Whether to install Busybox into the system environment as a very low
+      priority fallback for common commands. This should *never* override a
+      user-installed package.
+    '';
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages = with pkgs; lib.mkOrder 50 [
+      busybox
+    ];
+  };
+}

system/common/default.nix

@@ -6,5 +6,6 @@
     ./wm.nix
     ./nix.nix
     ./bootnext.nix
+    ./busybox.nix
   ];
 }

system/profile/base.nix

@@ -16,6 +16,8 @@ in
         me.enable = lib.mkDefault true;
         # Enable system Nix configuration
         nix.enable = lib.mkDefault true;
+        # busybox low-priority install
+        busybox.enable = lib.mkDefault true;
       };
 
       # locale settings