busybox: enable arp applet

It's pretty common, small and useful, specially since busybox ip doesn't
do neigh (and looking at /proc/net/arp is all fine but not very
friendly).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>