tools: kwbimage: Reject mkimage -F option

mkimage -F option (re-sign existing FIT image) signaled by fflag is not
supported by kwbimage. So mark its usage as invalid parameter.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/tools/kwbimage.c b/tools/kwbimage.c
index 97be3be..0c3b40d 100644
--- a/tools/kwbimage.c
+++ b/tools/kwbimage.c
@@ -2440,7 +2440,7 @@
 	}
 
 	return (params->dflag && (params->fflag || params->lflag)) ||
-		(params->fflag && (params->dflag || params->lflag)) ||
+		(params->fflag) ||
 		(params->lflag && (params->dflag || params->fflag)) ||
 		(params->xflag);
 }