mkimage: Allow updating the FIT timestamp

Normally the FIT timestamp is created the first time mkimage is run on a
FIT, when converting the source .its to the binary .fit file. This
corresponds to using the -f flag. But if the original input to mkimage is
a binary file (already compiled) then the timestamp is assumed to have
been set previously.

Add a -t flag to allow setting the timestamp in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/imagetool.h b/tools/imagetool.h
index f54809c..acbc48e 100644
--- a/tools/imagetool.h
+++ b/tools/imagetool.h
@@ -81,6 +81,7 @@
 	unsigned int external_offset;	/* Add padding to external data */
 	int bl_len;		/* Block length in byte for external data */
 	const char *engine_id;	/* Engine to use for signing */
+	bool reset_timestamp;	/* Reset the timestamp on an existing image */
 };
 
 /*