create-spdx: Use SPDXAnnotation to track native recipes

Create a small function that checks for 'isNative' as part of an Annotation

When the collect_dep_sources() runs, it collects sources from both native
and non-native recipes. Later when the GENERATED_FROM matching occurs it
may find the file (via checksum) from the native recipe since it's the
same checksum as the target file. The that are generated DocumentRefs
point to the native recipe rather than the target recipe DocumentRef.

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