net: Add Ethernet support to the network stack

This patch adds support for Ethernet to the network stack, which
entails relying on the Ethernet device driver to set the MAC address,
invoking ARP routines at the appropriate points, configuring the
Contiki network stack to use the appropriate link-level header length,
etc.

Change-Id: I153a6f4c83c28ca7129ef5affa1e9a18b0f92913
Signed-off-by: Michael LeMay <michael.lemay@intel.com>