updating sources from remote trees

Sources from remote trees creates a problem because some sources use repo/git to separate out projects into individual repos and then they are combined into a single large project using Repo (python wrapper around git to manage multiple repositories much more cleanly than git subtrees)

Other sources (mainly from china) dispense with the complications of managing multiple repos and simply create a single repo with all the source as one project. This is easier for them to maintain, but it creates a level of distance from the original source and it's updates. This allows the maintainers to make changes throughout the code base while protecting them from the complexities of merging updates.

We are caught in the middle. Our devices would be more easily managed if projects could be shared among multiple devices from different manufacturers, but any updates we make in the core source can't easily be managed in this fashion. In order to pull updates from the source, our project tree must include the source as it exists in the remote repo (by device mfr). If we use repo, then the changed source must be merged from the monolithic project to the disparate projects, preferrably with the commit history intact.