[PATCH dtc] Add -o <output file> to the usage message.

[Jon Loeliger]

So, like, the other day Jerry Van Baren mumbled:
> 
> Signed-off-by: Gerald Van Baren <vanbaren at cideas.com>
> ---
> 
> It turns out -o <file> was there all along, just not in the usage. :-)

Applied.

Thanks,
jdl