siggen/runqueue/bitbake-worker: Improve siggen data transfer interface

We need to transfer some of the siggen data from the core/cooker into
the worker instances. There was a partial API created for this but
its ugly and its not possible to extend it from the siggen class.

This patch completes the interface/abstraction for the data and
means the class can extend/customise it in any siggen class.

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