kconfig/nconf: use the generic menu_get_ext_help()

nconf is the only front-end which does not use this helper, but prefer
to copy/paste the code. The test wrt. menu validity added in this
version of the code is bogus anyway as an invalid menu will get
dereferenced a few line below by calling menu_get_prompt().

For now, convert nconf to use menu_get_ext_help(), as do every other
front-end. We will deals with menu validity checks properly in a
separate commit.

Cc: Nir Tzachar <nir.tzachar@gmail.com>
Cc: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>