Cleanup Oryx terminology
The term "source" is used in oryx-apps to refer to a location from which guest images can be obtained. This can easily lead to a bit of confusion as we already have the Oryx "sources" (Oryx repository, bitbake and Yocto Project layers) and the "sources" for each recipe (gcc sources, binutils sources, etc). Perhaps we should use "feed" instead in oryx-apps, this would match the existing
There may be other bits of terminology which are unclear so we should do a full review and be prepared to change things to improve clarity where needed.