kconfig/lxdialog: let <ESC><ESC> behave as expected

<ESC><ESC> is used to step one back in the dialogs.
When lxdialog became built-in pressing <ESC> once would cause one step back
and pressing <ESC><ESC> would cause two steps back.
This patch - based on concept from Roman Zippel <zippel@linux-m68k.org> -
makes one <ESC> a noop and pressing <ESC><ESC> will cause one step backward.

In addition the final yes/no dialog now has the option to go back to the
the kernel configuration. So if you get too far out you can now go back
to configuring the kernel without saving and starting all over again.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>