support: update Kconfiglib to the latest version

Corresponds to a95f477 in https://github.com/ulfalizer/Kconfiglib

.

Fixes:

 - Unset user values when loading a zero-byte .config. (5e54e2c)

 - Ignore indented .config assignments. (f8a7510)

 - Do not require $srctree to be set for non-kernel projects. (d56e9c1)

 - Allow digits in $-references to symbols. (ecacdd5)

 - Add Symbol.is_allnoconfig_y(). (deaa624)

 - Fix small output issue with Comments inside Choices.

Also adds Python 3 support and has a lot of internal cleanup and
optimization.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>