poplar: use random mac address

Set CONFIG_NET_RANDOM_ETHADDR=y, which sets random eth address in
case there is no configuration provided neither in CONFIG_ETHADDR
nor in "ethaddr" env variable.

This fixes the problem:
poplar# dhcp
Error: ethernet@9841000 address not set.

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>