tools: image: allow to sign image nodes without -K option

If -K option is missing when you sign image nodes, it fails with
an unclear error message:

  tools/mkimage Can't add hashes to FIT blob: -1

It is hard to figure out the cause of the failure.

In contrast, when you sign configuration nodes, -K is optional because
fit_config_process_sig() returns successfully if keydest is unset.
Probably this is a preferred behavior when you want to update FIT with
the same key; you do not have to update the public key in this case.

So, this commit changes fit_image_process_sig() to continue signing
without keydest.  If ->add_verify_data() fails, show a clearer error
message, which has been borrowed from fit_config_process_sig().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

tools/image-host.c

@@ -242,18 +242,19 @@ static int fit_image_process_sig(const char *keydir, void *keydest,
 	/* Get keyname again, as FDT has changed and invalidated our pointer */
 	info.keyname = fdt_getprop(fit, noffset, "key-name-hint", NULL);
 
-	if (keydest)
-		ret = info.crypto->add_verify_data(&info, keydest);
-	else
-		return -1;
-
 	/*
 	 * Write the public key into the supplied FDT file; this might fail
 	 * several times, since we try signing with successively increasing
 	 * size values
 	 */
-	if (keydest && ret)
-		return ret;
+	if (keydest) {
+		ret = info.crypto->add_verify_data(&info, keydest);
+		if (ret) {
+			printf("Failed to add verification data for '%s' signature node in '%s' image node\n",
+			       node_name, image_name);
+			return ret;
+		}
+	}
 
 	return 0;
 }