spdx30_tasks: Exclude 'doc' when exporting PACKAGECONFIG to SPDX

spdx30_tasks: Exclude 'doc' when exporting PACKAGECONFIG to SPDX

Currently when generating an SBOM, all packages have the 'doc' feature
indicated as disabled. This is in fact *not* a feature that was
declared in the recipe, but instead the documentation of the
PACKAGECONFIG variable.

But to be safe, if somehow a feature is named 'doc' and enabled, do
not exclude it when exporting PACKAGECONFIG features to SPDX.

Signed-off-by: Benjamin Robin (Schneider Electric) <benjamin.robin@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>