cmake: introduction of ZephyrBuildConfiguration package

The ZephyrBuildConfiguration package allow downstream users to control
the Zephyr build system using a cmake package.

A Zephyr Build Configuration package allows for setting of additional
DTS_ROOT, BOARD_ROOT, and similar variables without having to patch
Zephyr repo, but it also allows for inclusion of additional boilerplate
code for more advanced use cases.

The repository or folder containing the Zephyr Build Configuration
package must be on toplevel in the Zephyr workspace.

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