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>

package/busybox/busybox.config

@@ -767,7 +767,7 @@ CONFIG_FEATURE_FANCY_PING=y
 # CONFIG_FEATURE_UNIX_LOCAL is not set
 # CONFIG_FEATURE_PREFER_IPV4_ADDRESS is not set
 # CONFIG_VERBOSE_RESOLUTION_ERRORS is not set
-# CONFIG_ARP is not set
+CONFIG_ARP=y
 CONFIG_ARPING=y
 # CONFIG_BRCTL is not set
 # CONFIG_FEATURE_BRCTL_FANCY is not set