cmake: ZephyrUnittestConfig.cmake added

Adding ZephyrUnittestConfig.cmake and ZephyrUnittestConfigVersion.cmake
to allow unittest projects to use find_package to locate ZephyrUnittest.

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.

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