mkimage: Allow updating the FIT timestamp

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>