nconfig: Fix help for choice menus

When getting the help for a choice menu with a help section (for example
the "Choose SLAB allocator" menu) nconfig pops up a window with nothing
inside it. This is due to show_help() passing an empty string to
show_scroll_win()'s 3rd argument. The option really does have help
though, but it isn't a config symbol, so just add the help text for
the option, and don't try to add anything else like the config option
name.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Acked-by: Nir Tzachar <nir.tzachar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>