mkimage: fit: Fix signing of configs with external data

Just like we exclude data-size, data-position, and data-offset from
fit_config_check_sig, we must exclude them while signing as well.

While we're at it, use the FIT_DATA_* defines for fit_config_check_sig
as welll.

Fixes: 8edecd31 ("fit: Fix verification of images with external data")
Fixes: c522949a ("rsa: sig: fix config signature check for fit with padding")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>