expo: Update bootflow_menu_poll() to return a sequence ID

expo: Update bootflow_menu_poll() to return a sequence ID

Rather than returning a bootflow, return the index of the bootflow. This
will allow callers to do their own translation to bootflows or some
other data structure.

Also return a special code when the user tries to move the pointer, so
that the caller can cancel the boot-menu timeout, if this is in use.

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