README.kconfig: document backward compatibility "make *_config"

Commit 3ff291f3
(kconfig: convert Kconfig helper script into a shell script)
restored "<board>_config" target for backward compatibility.
It should be documented.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>