Additional layer(s) with prepackaged RTOS?

After talking with @gcloughley, I wonder if we should have an additional layer on top of the existing stack that also provides a snapshot of the RTOS and a sensible setting for the `CHERIOT_RTOS_SDK` environment variable we tend to use for `xmake` `include()`-s. It'd remove the need for users to separately clone the RTOS before building their application.

If that seems like a good idea, we might go a step further as well and add an additional layer on top of that with additional board support, either beside the RTOS or landed directly into the RTOS `sdk/boards` and `sdk/includes/platform` directories?)