mkimage: Add -c option to specify a comment for key signing

When signing an image, it is useful to add some details about which tool
or person is authorising the signing. Add a comment field which can take
care of miscellaneous requirements.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

tools/mkimage.h

@@ -89,6 +89,7 @@ struct mkimage_params {
 	char *cmdname;
 	const char *keydir;	/* Directory holding private keys */
 	const char *keydest;	/* Destination .dtb for public key */
+	const char *comment;	/* Comment to add to signature node */
 };
 
 /*