[Buildroot] New version of Kconfiglib available

[Ulf Magnusson]

Hello,

I've done lots of internal cleanup and optimization and fixed a few small
bugs/incompatibilities in Kconfiglib (https://github.com/ulfalizer/Kconfiglib):

 - 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)

Python 3 is supported now as well.

It should be a quick copy-paste update of kconfiglib.py. Just to be paranoid, I
verified that gen-manual-lists.py generates the same output with the new
version. It might run slightly faster too, though I guess that isn't very
important here. :P

https://github.com/ulfalizer/Kconfiglib/commit/d56e9c1d4c3e6f560382ac71fa2419b2a9bef1cb
summarizes of the last bug. There's no need to set $srctree in
gen-manual-lists.py if it isn't referenced in the Kconfigs. gen-manual-lists.py
could just do

self.config = kconfiglib.Config(os.path.join(self.base_dir,
                                             self.root_config),
                                base_dir = self.base_dir)

(That works with the old version too.)

I haven't run into the first two bugs in the wild by the way.

Cheers,
Ulf