Reverse the meaning of the fit_config_verify() return code

It is more common to have 0 mean OK, and -ve mean error. Change this
function to work the same way to avoid confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/image-sig.c

@@ -467,6 +467,6 @@ int fit_config_verify_required_sigs(const void *fit, int conf_noffset,
 
 int fit_config_verify(const void *fit, int conf_noffset)
 {
-	return !fit_config_verify_required_sigs(fit, conf_noffset,
-						gd_fdt_blob());
+	return fit_config_verify_required_sigs(fit, conf_noffset,
+					       gd_fdt_blob());
 }