openvpn: bypass check for /sbin/ip on the host

In the commit 'openvpn: use default iproute2 path', the configure flag
to explicitly set the iproute2 path was removed, since busybox now
provides the 'ip' applet at the default path.  However, setting this
flag is necessary to bypass the configure-time check for /sbin/ip on the
host, which will otherwise fail if iproute2 is not installed on the
host.  Add back the flag (pointing to the correct path), and add a
comment to describe why this is necessary.

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