rpi-u-boot-scr: WORKDIR -> UNPACKDIR transition

[leon-anavi]

This adapts to the oe-core rework to enforce a separate directory for unpacking local sources (UNPACKDIR) instead of directly using WORKDIR.

Follows the preliminary guideline from: https://lists.openembedded.org/g/openembedded-architecture/message/2007

- What I did

Fixed issues with the S variable in newer Yocto and OpenEmbedded versions.

- How I did it

By following the guideline: https://lists.openembedded.org/g/openembedded-architecture/message/2007