expo: Add a configuration editor

Add a new 'cedit' command which allows editing configuration using an
expo. The configuration items appear as menus on the display.

This is extremely basic, only supporting menus and not providing any way
to load or save the configuration.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 388e59f..f2ffd27 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -39,6 +39,7 @@
    cmd/bootz
    cmd/cat
    cmd/cbsysinfo
+   cmd/cedit
    cmd/cls
    cmd/cmp
    cmd/coninfo