kconfig: Move Kconfig.modules to the root build directory

Currently, the Kconfig.modules file is placed in the build directory
relative to the CMake "project". But technically, the file is not
project-specific, but global, or build-directory specific.

So we move it up one level to the CMAKE_BINARY_DIR instead. Currently,
there is only one project, so this change has no effect, but this
enables us to have multiple projects in the future, which again
enables multi-image builds.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>