tools/genboardscfg.py: improve performance more with Kconfiglib

The idea of using Kconfiglib was given by Tom Rini.
It allows us to scan lots of defconfigs very quickly.
This commit also uses multiprocessing for further acceleration.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Suggested-by: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>