create-spdx-3.0/populate_sdk_base: Add SDK_CLASSES inherit mechanism to fix tarball...

create-spdx-3.0/populate_sdk_base: Add SDK_CLASSES inherit mechanism to fix tarball SPDX manifests

Currently, "tarball" sdk based recipes don't generate SPDX manifests as they
don't include the rootfs generation classes. Split the SPDX 3.0 image class into
two so the SDK components can be included where needed.

To do this, introduce an SDK_CLASSES variable similar to IMAGE_CLASSES which
the SDK code can use.

Migrate testsdk usage to this.

Also move the image/sdk spdx classes to classes-recipe rather than the general classes
directory since they'd never be included on a global level.

For buildtools-tarball, it has its own testsdk functions so disable the class there as
a deferred inherit would overwrite it.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>