toradex: common: unify behaviour when config block is missing

If the config block is missing, various things may fail or behave
strangely on certain modules. This patch unifies that behaviour by
using a fake MAC address, until user updates the config block.

Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>