check-config: fix wrong comment about how to build whitelist

The command suggested in this comment block is wrong; it would not
rip off CONFIG options that had already been converted to Kconfig.

Instead, we should use the scripts/build-whitelist.sh tool.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>