openvpn: improved to let configure find the 'ip' command

Note:

This is particulary needed when cross-compiling openvpn but is also
useful if a custom prefix is used for iproute2.

network/openvpn/openvpn.conf

@@ -2,7 +2,7 @@
 # This copyright note is auto-generated by ./scripts/Create-CopyPatch.
 #
 # Filename: package/.../openvpn/openvpn.conf
-# Copyright (C) 2007 - 2011 The OpenSDE Project
+# Copyright (C) 2007 - 2013 The OpenSDE Project
 #
 # More information can be found in the files COPYING and README.
 #
@@ -42,7 +42,10 @@ var_append confopt ' ' "--enable-password-save"
 var_append CFLAGS ' ' "-DPLUGIN_LIBDIR${plugin_libdir}"
 
 # iproute2 support
-pkginstalled iproute2 && var_append confopt ' ' "--enable-iproute2"
+if pkginstalled iproute2; then
+	var_append confopt ' ' "--enable-iproute2"
+	var_append configprefix ' ' "IPROUTE=$( pkgprefix -r bindir iproute2 )"
+fi
 
 if ! pkginstalled lzo ; then
 	var_append confopt ' ' "--disable-lzo"