cmake: create a kernel.cmake CMake module

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