Resource copying method in samples is inconsistent

[thrimbor]

The samples currently use multiple different methods to copy additional resources required by the sample. The approach taken by the SDL_ttf sample (introduced in #89) essentially hijacks the default rule to execute a simple copy command. This has the drawback of not updating the copy when the resource changes, as well as not cleaning up properly on make clean (it also includes a potential race condition because ISO creation might run in parallel to the copy rule). Another approach is taken by #319, which uses a generic copying rule and injects the copy of the resource as a dependency for ISO-generation. It also adds the copy of the resource into the TARGET list to make the sample clean up properly when executing make clean.

These two (and maybe more) methods should get properly evaluated and we should settle for a single approach for the codebase.