kconfig: don't silently ignore unhandled characters

At the very least we should tell people that what they wrote is not
what the utility understands.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michal Marek <mmarek@suse.cz>