cmake: relocation of generated Kconfig file for Zephyr modules

This commit relocates the generated Kconfig.modules file from
${CMAKE_BINARY_DIR} into ${CMAKE_BINARY_DIR}/Kconfig.

This is done as preparation of multiple SOC and ARCH root support.

A single Kconfig folder inside ${CMAKE_BINARY_DIR} helps to keep the
build folder tidy.

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