[U-Boot] Kconfiglib updates

[Ulf Magnusson]

Hello,

I've done lots of internal cleanup and optimization and fixed a few small
bugs/incompatibilities in 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 (+ add back header) update. Just to be
paranoid, I verified that genboardscfg.py generates an identical boards.cfg
with the new version. Seems to be slightly faster too, though I guess it's
fast enough already. :)

https://github.com/ulfalizer/Kconfiglib/commit/d56e9c1d4c3e6f560382ac71fa2419b2a9bef1cb
has a summary of the last bug. No need to set $srctree in genboardscfg.py now
if it isn't referenced in the Kconfigs, but doing

self._conf = kconfiglib.Config(base_dir = ".")

would be safest if the base directory should always be the current directory
regardless of the value of $srctree. The special $srctree handling is to get
alternative kernel build directories working with minimal fuss.

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

Cheers,
Ulf