menu: menu_get_choice() return -ENOENT if menu item is empty

menu_get_choice() needs to handle the case that menu item
is empty. In this case, menu_get_choice() returns -ENOENT.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>