fix busy box dhcp bug which will result in an empty /etc/resolv.conf

bug in the configuration of busy box setting the /etc/resolv.conf to an empty value if there is no name server data coming from the DHCP Server for a second network interface #93 .

Please check if your tiny core linux already includes this fix:
https://bugs.busybox.net/show_bug.cgi?id=6788

```
/usr/share/udhcpc/default.script
@@ -28,7 +28,9 @@ case "$1" in
             done
         fi

-        echo -n > $RESOLV_CONF
+        if [ -n "$dns" ] ; then
+            echo -n > $RESOLV_CONF
+        fi
         [ -n "$domain" ] && echo search $domain >> $RESOLV_CONF
         for i in $dns ; do
             echo adding dns $i
```