kconfig: add menu_get_ext_help function to display more information

The three functions are moved from mconf.c, then they can be shared in
all menuconfig & gconfig & xconfig & config.

  +void menu_get_ext_help(struct menu *menu, struct gstr *help)
  +static void get_prompt_str(struct gstr *r, struct property *prop)
  +void get_symbol_str(struct gstr *r, struct symbol *sym)

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