cmake: boards and shields cmake files repurposed

The boards.cmake and shields.cmake has been repurposed to do board and
shield validation so that such validation code can be re-factored out
of boilerplate itself.

The boards.cmake and shields.cmake will both perform validation and
printing errors when validation fails.

Also it will specify the boards and shields build targets.

This ensures that validation code, message printing and target
specification for boards and shields are located logically together.

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>