We have three kinds of operating system trees, until we unify them to one,
hide them behind one interface. Use this to read the architecture from the
Tree rather than pass it in as a string to parent pipelines.
Also, make the filename parameter optional in a few places, there should be no
reason to set this rather than introspect it (except for backwards
compatibility).
Lastly, add another playground example sample to build a raw image.
