* onnxifi: add 'onnx' as external project
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnxifi: public interface function stubs
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnxifi: fix the typo in function name
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix compilation Ubuntu 16.04
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnxifi: update to dummy backend
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnxifi: style cleanup
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>