#29 TODO: support multiple payload versions for different boards

For example, a certain revision of a given payload might not play nicely with a given version of coreboot. At present, the retroboot build system uses the same revision of a given payload for all boards, while allowing different coreboot revisions. In practise, efforts are going to be made to keep all mainboards mostly on the same coreboot revision (or near each other) so this won't be an issue all of the time. However, it is an issue that needs to be fixed.