cmake: Added assert that will show usage

There is a catch-22 in that you need to run CMake to get a build
system that can show you usage, but you need to know the usage to be
able to get a build system.

This assert could be used to improve the usability somewhat. When
invalid usage is detected it can be used to print the usage.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>