arm: mvebu: turris_omnia: Define CONFIG_ETHPRIME instead of ethact= ENV

CONFIG_ETHPRIME defines primary ethernet device and env variable $ethact
stores currently active ethernet device.

So there is no point to set ethact= in default environment. Instead set
CONFIG_ETHPRIME properly.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>