kconfiglib: Update to default to UTF-8 for Python 3

Update Kconfiglib to upstream revision da40c014398f3 (+ local Zephyr
modifications) to get commit da40c014398f3 ("Force encoding to UTF-8 by
default on Python 3") in. It sets a (configurable) UTF-8 default for
Python 3, overriding the encoding specified in the current locale.

I've decided that this is a good idea after some problem reports
unrelated to Zephyr. Running with the C locale breaks things horribly
otherwise, and the fix isn't obvious.

Plain strings aren't decoded on Python 2, so no changes are needed
there.

Related PEP: https://www.python.org/dev/peps/pep-0538/



Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>