kconfig: Decode Kconfig sources as UTF-8 instead of as LC_CTYPE

Decode Kconfig sources as UTF-8 instead of decoding them according to
the system locale (which might be ascii-only).

This resolves an issue that can be reproduced like this:

$ export LANG=C
$ echo '# Sebastian Bøe' >> ~/zephyr/Kconfig
$ build_any_zephyr_app

See the Python comment for details.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>