tools/genboardscfg.py: improve performance more by using Kconfiglib

This commit makes the tool much faster.

Kconfiglib is needed to try this commit.

Fetch and install it as follows (need to be root for installation)

 $ git clone git://github.com/ulfalizer/Kconfiglib.git
 $ cd Kconfiglib
 # python setup.py install

Or more simply, you can download it into tools/ directory:

 $ cd tools
 $ wget https://raw.githubusercontent.com/ulfalizer/Kconfiglib/master/\

I hesitate to add Kconfiglib directly to U-Boot's repository.
U-Boot is distributed under GPL license, whereas Kconfiglib
claims ISC license.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>