doc: Mention CONFIG_DEFAULT_ENV_FILE

Add mention of this option since it does a similar thing to the text
environment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Marek Behún <marek.behun@nic.cz>