Adding option -Coutput_dir= to control output location.

- If not specified, will be initialized with dirname(test_desc)
- Like -Cflatbuffer_dir, reference model isn't responsible for creating directory.
  User need to make sure target directory path exists.

Signed-off-by: Kevin Cheng <kevin.cheng@arm.com>
Change-Id: I9cfcc801cff648e53306f8de8ea8d5eaaf87ea80