cmake: create a root.cmake CMake module

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