west: sign.py: recommend separator -- tool_args always

In the following command, the first argument `for_rimage` is passed to
`rimage` whereas `--for west` goes to west.

```
west sign  for_rimage --for west
```

This is somehow valid but we really don't want anyone to do that.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
1 file changed