cmake: Zephyr package now sets ZEPHYR_BASE as cmake cached variable

This commit introduces ZEPHYR_BASE as a cached variable, ensures that
once ZEPHYR_BASE has been set in a project, it is sticky.
Using cached variable also allows users to reconfigure ZEPHYR_BASE if
they so wish.

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