cmake: zephyr module processing improvements.

This commit improves Zephyr modules support in CMake.
All Zephyr modules will now have a corresponding
`ZEPHYR_${module_name}_MODULE_DIR` set, regardless of whether they
define a cmake folder in module.yml.

This results in:
`ZEPHYR_${module_name}_MODULE_DIR` defined for all modules and refers to
the root of the modle.
`ZEPHYR_${module_name}_CMAKE_DIR` defined for modules that specifies
cmake in module.yml, or is having both a CMakeLists.txt and Kconfig file
in the zephyr folder.

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