expo: Add a function to poll for input

expo: Add a function to poll for input

Both bootflow_menu and cedit use similar logic to poll an expo. Move
this into the expo library so the code can be shared.

Update bootflow_menu_run() to return -EPIPE when the user quits without
choosing anything, since -EAGAIN is ambiguous and elsewhere means that
there is no input yet.

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