expo: Begin implementation of a text editor

It is useful to be able to edit text, e.g. to allow the user to edit the
environment or the command-line arguments for the OS.

Add the beginnings of an implementation. Future work is needed to finish
this: keypress handling and scrolling. For now it just displays the
text.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/expo.rst b/doc/develop/expo.rst
index d6fc487..b94340e 100644
--- a/doc/develop/expo.rst
+++ b/doc/develop/expo.rst
@@ -560,6 +560,7 @@
 Some ideas for future work:
 
 - Default menu item and a timeout
+- Complete the text editor
 - Image formats other than BMP
 - Use of ANSI sequences to control a serial terminal
 - Colour selection