cmake: extend zephyr_get() to handle build configurations from sysbuild

Enhance sysbuild controlled configurations.

The current scheme of passing settings using `-D` on the CMake
invocation is vulnerable to quoting and lists.

With `zephyr_get()` in place as a uniform way of handling user
controlled settings (CMake cache / environment / CMake local variable)
we have a mechanism in place for a cleaner handling of sysbuild
controlled settings.

This improves the robustness of variable passing and add the same cleans
up and simplifies the logic in sysbuild.

The Kconfig Zephyr CMake module has been updated accordingly so that
CONFIG settings are taken from the sysbuild shadow cache when sysbuild
is used as higher level build system.

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