It's not related to a project but general support for computer-on-module (COM) boards. COM board is a single-board computers containing CPU, Memory, Chipset (SoC). These boards don't contain the standard connectors for I/O. These boards need to be placed on a carrier board (baseboad), which provides the standard I/O connectors and the power.
Thanks, sorry, I really misunderstood what this was about then. In this case, if the same baseboard can be used with different SOMs (like Nico said), I agree that adding a different toplevel directory makes sense. (Otherwise, the variant approach should be good enough.)