A firmware compiled with BUSYBOX_CONFIG_ARP should also use by default the
arp binary from busybox. Otherwise the extra functionality the user
requested can only be used when running arp with the path to the binary.

Signed-off-by: Marek Lindner <marek.lindner@open-mesh.com>
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>