kconfig: tristate choices with mixed tristate and boolean values

Change kconfig behavior so that mixing bool and tristate config settings in
a choice is possible and has the desired effect of offering just the
tristate options individually if the choice gets set to M, and a normal
boolean selection if the choice gets set to Y.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>