README.kconfig: document backward compatibility "make *_config"

Commit 3ff291f371fa9858426774f3732924bacb61ed1c
(kconfig: convert Kconfig helper script into a shell script)
restored "<board>_config" target for backward compatibility.
It should be documented.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Series-version: 2
Series-changes: 2
  - Update the commit description

Series-cc: trini
Series-cc: sjg
Series-cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Series-cc: Stephen Warren <swarren@nvidia.com>
tools: moveconfig.py
Do not send: Kconfiglib

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
moveconfig2
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>