scripts/check-config.sh: fix to be compatible with BSD sed

Fist use extended regexp in order to drop the '\' around the
parentheses which is not supported by BSD sed in regular mode.

Secondly use [[:blank:]] instead of \s, as the later is a GNU
extension.

No functional change intended.

Signed-off-by: Roger Pau Monné <royger@FreeBSD.org>
Reviewed-by: Simon Glass <sjg@chromium.org>