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>

diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
index 043c02d9a94edd3b3c3e779f15f33ea1914e1199..6f3066e2b654240b9562f6f4e36e16052365570d 100644 (file)
--- a/doc/usage/environment.rst
+++ b/doc/usage/environment.rst
@@ -458,3 +458,18 @@ The signature of the callback functions is::
   include/search.h
 
 The return value is 0 if the variable change is accepted and 1 otherwise.
+
+
+External environment file
+-------------------------
+
+The `CONFIG_USE_DEFAULT_ENV_FILE` option provides a way to bypass the
+environment generation in U-Boot. If enabled, then `CONFIG_DEFAULT_ENV_FILE`
+provides the name of a file which is converted into the environment,
+completely bypassing the standard environment variables in `env_default.h`.
+
+The format is the same as accepted by the mkenvimage tool, with lines containing
+key=value pairs. Blank lines and lines beginning with # are ignored.
+
+Future work may unify this feature with the text-based environment, perhaps
+moving the contents of `env_default.h` to a text file.