cmake: add a mechanism for deprecating modules

We currently have mechanisms for deprecating both boards and SoCs.
However, we lack one for deprecating modules. This is inconvenient,
because we would like to do exactly that.

Handle this in a simple way by adding a new CMake file in the modules
directory which is responsible for warning the user about any
deprecated modules they may be using. See the source code comments for
more details about the approach.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>