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

On Espressobin board value for $fdtfile cannot be determined at compile
time and is calculated at board runtime code. This change uses a new option
DEFAULT_ENV_IS_RW to allow modifying default_environment[] array at runtime
and set into it correct value.

This change also ensure that 'env default -a' set correct value to $fdtfile.

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