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>
diff --git a/doc/mkimage.1 b/doc/mkimage.1
index f9c733a..b67a351 100644
--- a/doc/mkimage.1
+++ b/doc/mkimage.1
@@ -97,6 +97,12 @@
 .B Create FIT image:
 
 .TP
+.BI "\-c [" "comment" "]"
+Specifies a comment to be added when signing. This is typically a useful
+message which describes how the image was signed or some other useful
+information.
+
+.TP
 .BI "\-D [" "dtc options" "]"
 Provide special options to the device tree compiler that is used to
 create the image.