km/boards: add MAC address offset as argument to ivm_populate_env

For upcoming SECU board we have different MAC address offsets depending
on which functional unit we are running. In this case we need to pass
this value to the ivm_populate_env function instead of using the hard
coded config option there.

Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
CC: Stefan Roese <sr@denx.de>
CC: Valentin Longchamp <valentin.longchamp@ch.abb.com>
CC: Marek Vasut <marex@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>