Unprivileged scheduler

It should be possible for a board implementation to setup hardware protection such that the scheduler runs in unprivileged mode while the board stays privileged. This means that when the scheduler calls into the board API, the implementation (controlled by the board) should escalate to privileged mode (if needed). The end goal being to reduce the amount of code running in privileged mode. It is not clear yet if the board API needs any type of support for this use case.