create-spdx-2.2: Add support for custom Annotations

create-spdx-2.2: Add support for custom Annotations

This change adds a new variable to track which recipe variables
are added as SPDX Annotations.

Usage: add SPDX_CUSTOM_ANNOTATION_VARS = <some recipe variable>

The recipe spdx json will contain an annotation stanza that looks
something like this:

     "annotations": [
        {
          "annotationDate": "2023-02-13T19:44:20Z",
          "annotationType": "OTHER",
          "annotator": "Tool: oe-spdx-creator - 1.0",
          "comment": "CUSTOM_VARIABLE=some value or string"
        },

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