tools: add genboardscfg.py

Now the primary data for each board is in Kconfig, defconfig and
MAINTAINERS.

It is true boards.cfg is needed for MAKEALL and buildman and might be
useful to brouse all the supported boards in a single database.
But it would be painful to maintain the boards.cfg in sync.

So, this is the solution.
Add a tool to generate the equivalent boards.cfg file based on
the latest Kconfig, defconfig and MAINTAINERS.

We can keep all the functions of MAKEALL and buildman with it.

The best thing would be to change MAKEALL and buildman for not
depending on boards.cfg in the future, but it would take some time.

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