cmake: Using symlinks on unix like os'es for dependencies

Fixes: #8380

This commit fixes the side-effect of PR #8211 where a 'ninja clean'
would try to remove dependency folders.
Changes:
- Symlinks are created during build and CMake targets now depends on
  the symlinks. Thus, same behavior is achived with regards to
  dependency handling, while at the same time, the output can be
  cleaned as the dependencies are now attached to the symlinks.
- Dependencies have been changed so that generation of json files
  depends on the trigger file and CMake depends upon the subdir txt
  file. This prevents additional CMake runs after a clean.

Signed-off-by: Torsten Rasmussen <torsten.rasmussen@nordicsemi.no>