km/common: force set ethaddr after reading IVM

The ethaddr is stored inside the inventory eeprom. During
boottime this value was read out and the ethaddr variable was
set. Previously this was only done if it ethaddr == NULL but
this is wrong for our ARM boards. Because ethaddr is at this
stage never NULL for ARM due to the random calculation of a
MAC address in mvgbe.c.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>