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.

Also fix scripts/kconfig/conf's handling of children of choice values -
there may be more than one immediate child, and all of them need to be
processed.

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