board: turris: Allow to specify first eth idx of first MAC address

Turris Omnia uses first MAC address from OTP for second ethernet interface.
Second MAC address for third interface and third MAC address for first
interface.

Other Turris routers do not have this rotate by one mapping. So add
function parameter for specifying id of the first ethernet interface.

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