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.

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 = [