image: Avoid -ENODATA in host tools

Unfortunately -ENODATA is not available in OpenBSD. Use -EBADMSG
instead, to indicate a missing timestamp.

Fixes: c5819701 image: Adjust the workings of fit_check_format()
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>