tools/genboardscfg.py: improve performance

I guess some developers are already getting sick of this tool
because it generally takes a few minites to generate the boards.cfg
on a reasonable computer.

The idea popped up on my mind was to skip Makefiles and
to run script/kconfig/conf directly.
This tool should become about 4 times faster.
You might still not be satisfied, but better than doing nothing.

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