kbuild: beautify the log of config whitelist check

Use the kbuild style log.

Prior to this commit:

./scripts/check-config.sh u-boot.cfg \
	./scripts/config_whitelist.txt . 1>&2

With this commit:

  CFGCHK  u-boot.cfg

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