env: Allow to set default_environment[] from board code via compile option DEFAULT_ENV_IS_RW

This change allows board code to modify default_environment[] array when
compile option DEFAULT_ENV_IS_RW is specified in board config file.

Some board default variables depend on runtime configuration which is not
known at compile time. Therefore allow to set default_environment[] array
as non-const and allow board code to modify it when it is needed.

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