cmake: introduction of ZephyrExtension package

The ZephyrExtension package allow downstream users to extend the current
Zephyr package.
A ZephyrExtension package allows for setting of additional DTS_ROOT,
BOARD_ROOT, and similar variables without having to patch Zephyr repo.

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

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