openvpn: adjust iproute2 path logic

As reported in bug #6164 when using busybox iproute2 openvpn can fail to
locate the 'ip' utility.
This was correct until busybox 1.21.0 was released since it changed the
location of /bin/ip to /sbin/ip (what iproute2 proper does).
So adjust the ip path accordingly.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Merge branch 'master' of git://git.buildroot.net/buildroot