ifupdown: bump to version 0.8.8

Also add a kludge switching the explicit invocation of the ip binary
from /bin to /sbin since that's where buildroot installs it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>