bootstd: Add a return code to bootflow menu

Return an error when the user does not select an OS, so we know whether
to boot or not.

Move calling of bootflow_menu_run() into a separate function so we can
call it from other places.

Expand the test to cover these cases.

Add some documentation also, while we are here.

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