doc: document 'config' command

Add a little documentation for the config command and its new ability
to filter the output.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/config.rst

@@ -0,0 +1,40 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. index::
+   single: config (command)
+
+config command
+==============
+
+Synopsis
+--------
+
+::
+
+    config [<str>]
+
+Description
+-----------
+
+The config command prints the `.config` file used when building U-Boot
+to the console. Note that the `.config` file is usually several 1000
+lines long.
+
+If the optional argument is given, only lines containing that string
+(case insensitively) are printed. That can be useful if one wants to
+check whether a specific option is enabled, or just to limit the
+output to the subsystem of interest.
+
+The `.config` file is stored inside the U-Boot binary in
+gzip-compressed format.
+
+Examples
+--------
+
+.. code-block:: bash
+
+    # Print the entire .config
+    config
+
+    # Print all lines related to pinctrl
+    config pinctrl