devtool: finish: add dry-run option

If you're not sure what changes devtool finish is going to make, or
you're not sure you're finished with your modifications, it is useful to
be able to see what devtool finish is going to do beforehand, so add
a -N/--dry-run option to make that possible.

(It's also very useful for debugging devtool finish itself.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>