menuconfig: fix a regression when canceling the prompt dialog at exit

This commit fixes a bug, while introducing a new one..

commit 7203ddbd4be9720649e47d756a001e0c7d7f8ae2
Author: Wang YanQing <udknight@gmail.com>
Date:   Thu Jan 12 11:31:32 2012 +0800

    menuconfig: let make not report error when not save configuration

Pressing ESC should cancel the yes/no dialog and return back to
the main menu, but not exit from menuconfig.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>