cmake: support ZephyrBuildConfiguration_ROOT for package location

Fixes: #35890

The current implementation of Zephyr build configuration CMake package
only allows the build configuration package to be placed inside a
Zephyr workspace.

This commit extends the usability by allowing users to locate the
Zephyr build configuration CMake package outside the Zephyr workspace
and then refer to the package using
`-DZephyrBuildConfiguration_ROOT=<path>`
`set(ZephyrBuildConfiguration_ROOT <path>)`

This allows users greater flexibility in their workspace layouts.

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