kokkos_generated_settings.cmake placement is inconsistent

[jgfouca]

The critical file, kokkos_generated_settings.cmake, which is key to any cmake package's use of kokkos, does not get put in a consistent place, which introduces confusion.

When you build and install with generate_makefile, the file lives here:
$prefix/kokkos_generated_settings.cmake

When you build and install with cmake, the file lives here:
$prefix/lib/cmake/Kokkos/kokkos_generated_settings.cmake

[jgfouca]

I don't have admin rights to assign myself, but I have agreed to take this issue.