toradex: common: Use random mac address if toradex config block is missing

Set CONFIG_NET_RANDOM_ETHADDR=y, which sets random eth address in
case there is no toradex config block programmed.
In case of missing toradex config block the generated mac address was
always the same, linked to serial number 0.
Use random generated mac address instead.

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>