Update to support yocto walnascar

[MrTarantoga]

Due to the renaming of WORKDIR to UNPACKDIR, I had to drop the support for all previous versions.

See https://docs.yoctoproject.org/next/migration-guides/migration-5.1.html#s-workdir-no-longer-supported
Tested on MrTarantoga/SpotyPee#22 (works on my machine test)
Refer issue #224

[tronical]

Ouch, dropping support for older versions is very painful :(. Is there no way of supporting both?
Worst case via a separate class?

[MrTarantoga]

Added a distinction of cases, if the supported version of the core layer is "walnascar" it uses UNPACKDIR other WORKDIR.

For future versions, it must be tested against a list of compatible versions. But without knowing the mandatory adaptions for future releases, the work to anticipate, I think, not worth it.

Tested on my project wit walnascar.