busybox

examples/udhcp/simple.script: add possibility to use modern "ip"

Script uses "ifconfig" only, not up-to-date so much.
This patch adds "ip" in condition if exists.

Signed-off-by: Jiří Prchal <jiri.prchal@aksignal.cz>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>