kconfig: remove mkconfig and boards.cfg

The old configuration script is no longer necessary.
Nor is boards.cfg a primary database.
We can generate it with the genboardscfg.py tool
based on the latest Kconfig, defconfig and MAINTAINERS.

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