cmake: export Zephyr modules as Kconfig variables.

In CMake it is possible to reference each Zephyr module as a CMake
variable. This commit introduces the same feature to Kconfig, so that
it is possible to reference a module in consistent way in both CMake
and Kconfig.

To include a file from module FOO in CMake, use:
include(${ZEPHYR_FOO_MODULE_DIR}/somefile.cmake

To source a file from module FOO in Kconfig, use:
source "$(ZEPHYR_FOO_MODULE_DIR)/Kconfig.somefile"

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