openvpn: update and enable iproute2 support

- Added .bb for version 2.3.4.
- The download URL was broken; it's changed to reflect where the website
  points to.
- Force use of iproute2, which is generally recommended when running
  OpenVPN as an unprivileged user. Ref:
  http://community.openvpn.net/openvpn/wiki/UnprivilegedUser


- Explicitly add libpam to DEPENDS if pam is enabled, and disable the
  auth-pam plugin if pam is not enabled.
- Pass the path to the 'ip' utility to the configure script to keep it
  from trying to find it on the host.

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>