cmake: kconfig: allow for Kconfig CONFIG namespace

This commit allows for specifying a dedicated Kconfig namespace, for
example: <namespace>_CONFIG

This namespace will then be used for handling loading of configuration
files into CMake configure scope, as well as handling of CMake cache
variables when set using `-D<namespace>_CONFIG_<var>=<value>`.

This allows greater flexibility when re-using the cmake/kconfig.cmake
in other projects.

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