cmake: adding ZephyrConfig.cmake to allow an easy way to locate Zephyr

Adding ZephyrConfig.cmake and ZephyrConfigVersion.cmake allows projects
to use find_package to locate Zephyr.

This means that it will be possible to allow users to run CMake without
the need to source zephyr-env.sh or run zephyr-env.cmd.

This is especially useful for IDEs such as Eclipse or SES, where it will
no longer be required to source the above files before launching the
IDE.

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