kconfig: Don't write invisible choice values

This makes it so "make oldconfig" really prompts for any choice where
options that previously weren't visible just became so. Previously one
would have to remember to go over all choice values and check whether
some that previously couldn't be selected now can be.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>