cmake: kconfig cmake file environment cleanup

This commit is a cleanup of the cmake/kconfig.cmake file.
It removes `set(ENV{<name>} <value>)` and instead creates a shared list
that can be used for execute_process() and add_custom_target() when
running Kconfig during CMake configure time or menuconfig and related
build targets.

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