Transitioning to Uptane

[jhdalek55]

Issue #4 asked how to deal with systems that have multiple image repositories. It received quite a bit of discussion initially, but nothing since April of 2021. After looking at the issue in our 2/14/23 meeting, we decided that this topic is probably better addressed as a use case in a larger examination of transition issues. As such, we decided to close the original issue and open this thread as a collection spot for topics and use cases relating to transitioning challenges when moving to Uptane from other systems.