docs: Zephyr module and CMake processing

This commit describes add a section which describes how Zephyr modules
are integrated into Zephyr build system and what Zephyr module variables
that are available during CMake and Kconfig processing.

Those variables may be used for refering other Zephyr modules or
including of additional code.

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