tools: dumpimage: Show error message when trying to extract data from kwbimage

There is no code for extracting data from kwbimage, so show an error
message when user tries this via e.g. dumpimage call:
  ./tools/dumpimage -T kwbimage -o /tmp/out u-boot-spl.kwb

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/tools/kwbimage.c b/tools/kwbimage.c
index 5410df6..9ecda86 100644
--- a/tools/kwbimage.c
+++ b/tools/kwbimage.c
@@ -1867,6 +1867,11 @@
  */
 static int kwbimage_check_params(struct image_tool_params *params)
 {
+	if (params->iflag) {
+		fprintf(stderr, "%s: kwbimage does not support extract operation\n", params->cmdname);
+		return CFG_INVALID;
+	}
+
 	if (!params->imagename || !strlen(params->imagename)) {
 		char *msg = "Configuration file for kwbimage creation omitted";