ddns: Allow running ddns using the /etc/init.d/ddns script

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

net/ddns-scripts/files/usr/lib/ddns/dynamic_dns_functions.sh

 	do
 		local iface
 		config_get iface "$section" interface "wan"
-		[ "$iface" = "$event_interface" ] || continue
+		[ -z "$event_interface" -o "$iface" = "$event_interface" ] || continue
 		/usr/lib/ddns/dynamic_dns_updater.sh $section 0 > /dev/null 2>&1 &
 	done
 }