arm: mvebu: Espressobin: Set default value for $ethNaddr env variable

On Espressobin board are MAC addresses stored in U-Boot env area. Therefore
they are not present in default_environment[] array constructed at compile
time.

This change puts permanent MAC addresses into default_environment[] array
at board runtime. Espressobin board has enabled DEFAULT_ENV_IS_RW option
and therefore can modify this array.

This change ensure that 'env default -a' does not delete permanent MAC
addresses from Espressobin env storage area.

Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Andre Heider <a.heider@gmail.com>