kconfig: make use of menu_get_ext_help in menuconfig

The removed functions are moved into menu.c for sharing with
gconfig & xconfig & config.

Signed-off-by: Cheng Renquan <crquan@gmail.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>