spdx.py: Add SPDXAnnotation Object

This is added to allow the create-spdx code to create annotations
that store values properly according to the SPDX Specification.

Initialy they will be used to track if a recipe is a native type.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>