armv8: ls2080a: Define CONFIG_ENV_OVERWRITE to overwrite serial and ethaddr

As the environment variables "serial#" and "ethaddr" need to be
overwriten by the users, CONFIG_ENV_OVERWRITE is defined to disable
the write protection. Anybody can change or delete these parameters.

Signed-off-by: Alison Wang <alison.wang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>