xconfig: fix 'Show Debug' functionality

xconfig - Fix missing 'Show Debug' functionality.

xconfig Help mentions 'Show Debug Info' but it was missing from any
menu.

* Add 'Show debug' menu to the main menu.
* Properly load showDebug settings.

Reported-by: Jason Vas Dias <jason.vas.dias@gmail.com>
Signed-off-by: Boris Barbulovski <bbarbulovski@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michal Marek <mmarek@suse.com>