board: sama5d2_xplained: Set 'ethaddr' got from AT24MAC

If 'ethaddr' is not set, we will get the ethernet address from AT24MAC,
and set it to 'ethaddr' variable.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Songjun Wu <songjun.wu@microchip.com>
Reviewed-by: Andreas Bießmann <biessmann@corscience.de>
2 files changed