env: sf: Allow to use env_sf_init_addr() at any stage

In some cases it makes sense to use env_sf_init_addr() also in SPL mode.
Allow it for boards by providing custom implementation of weak function
env_sf_get_env_addr(). When this function returns NULL it signals that
address is invalid, like config option CONFIG_ENV_ADDR.

There is no change in default behavior or in config options.

Signed-off-by: Pali Rohár <pali@kernel.org>