kconfig: fix choice multi-display

In case a choices has different combinations, it can be displayed
more than once.

See the discussion thread on the linux-kbuild ML:
http://www.spinics.net/lists/linux-kbuild/msg04709.html

The fix applied here was proposed by Arnaud LACOMBE.
That fix, or a similar one, will probably be pushed upstream soon.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 COPYING