Rebuild examples when platform changes

[mysterymath]

While the recent SDK refactoring helped with the NES ports, it caused the examples to no longer automatically rebuild when the underlying platform changes. This is because the examples refer to the platform only through the config files, which are invisible to CMake.

Given that this is akin to if the underlying C library changed in a regular CMake build, we should probably handle this outside of the example CMake config. We can create a stub file that regenerates each time any library or link.ld changes for a given target (or if there's a bleed-through from a parent target). Then, we can trigger a clean build of the corresponding example target whenever this stub file changes. Probably.