Improved diagnostic output: when the build fails because a package cannot be cloned into the build drive, it says which package and the repo source and destination. Improved caching logic, so that it doesn't depend on network availability as much. Improved indentation.

Made the generation of the syncing scripts more regular, and ensured that it all works and makes sense even when sf.spec.sourcesync.enable is false.

Updated PDK baseline for S^3 platform builds to newly released PDK_3.0.1