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/fit_image.c b/tools/fit_image.c
index a082d93..df310b5 100644
--- a/tools/fit_image.c
+++ b/tools/fit_image.c
@@ -53,7 +53,7 @@
 	}
 
 	/* for first image creation, add a timestamp at offset 0 i.e., root  */
-	if (params->datafile) {
+	if (params->datafile || params->reset_timestamp) {
 		time_t time = imagetool_get_source_date(params->cmdname,
 							sbuf.st_mtime);
 		ret = fit_set_timestamp(ptr, 0, time);