cmake: create a user_cache.cmake CMake module

The user_cache.cmake CMake module remove boilerplate code and place it
inside a dedicated user_cache.cmake CMake module.

The user cache functions has been moved to the new Zephyr CMake module.

This is part of a general CMake overhaul to allow better modularization
and reuse of the Zephyr build system.

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