expo: Provide access to the current menu item

expo: Provide access to the current menu item

Add functions to allow a caller to find out the current menu item and to
select a different one.

Update the event handling so that an attempt to change the current item
(e.g. by pressing the up-arrow key) is reported to the caller, since
this may be used to cancel an autoboot timeout.

Signed-off-by: Simon Glass <sjg@chromium.org>