iproute2: module to create rt_table file & co

When doing source routing/multihoming, it's practical to give names to routing
tables. The absence of the rt_table file in /etc make this impossible.
This patch recreates these files on rebuild so that they can be modified
by the user see NixOS#38638.

iproute2 is modified to look into config.networking.iproute2.confDir instead of
/etc/iproute2.

nixos/modules/config/iproute2.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.networking.iproute2;
+  confDir = "/run/iproute2";
+in
+{
+  options.networking.iproute2.enable = mkEnableOption "copy IP route configuration files";
+
+  config = mkMerge [
+    ({ nixpkgs.config.iproute2.confDir = confDir; })
+
+    (mkIf cfg.enable {
+      system.activationScripts.iproute2 = ''
+        cp -R ${pkgs.iproute}/etc/iproute2 ${confDir}
+        chmod -R 664 ${confDir}
+        chmod +x ${confDir}
+      '';
+    })
+  ];
+}

nixos/modules/module-list.nix

@@ -9,6 +9,7 @@
   ./config/fonts/ghostscript.nix
   ./config/gnu.nix
   ./config/i18n.nix
+  ./config/iproute2.nix
   ./config/krb5/default.nix
   ./config/ldap.nix
   ./config/networking.nix

pkgs/os-specific/linux/iproute/default.nix

@@ -1,4 +1,4 @@
-{ fetchurl, stdenv, lib, flex, bash, bison, db, iptables, pkgconfig }:
+{ fetchurl, stdenv, config, lib, flex, bash, bison, db, iptables, pkgconfig }:
 
 stdenv.mkDerivation rec {
   name = "iproute2-${version}";
@@ -26,8 +26,9 @@ stdenv.mkDerivation rec {
     "HDRDIR=$(TMPDIR)/include/iproute2" # Don't install headers
   ];
 
+  # enable iproute2 module if you want this folder to be created
   buildFlags = [
-    "CONFDIR=/etc/iproute2"
+    "CONFDIR=${config.iproute2.confDir or "/run/iproute2"}"
   ];
 
   installFlags = [