drivers: net: sandbox: add support for NET_LWIP

drivers: net: sandbox: add support for NET_LWIP

Make the sandbox mock ethernet driver (drivers/net/sandbox.c) compatible
with NET_LWIP by not relying on any of the structures or functions
defined in net-legacy.h. This is done by providing local definitions of
the various protocol structures (Ethernet, ARP, IPv4, ICMP). Drop the
stub driver that was introduced specifically for NET_LWIP
(drivers/net/sandbox-lwip.c).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>