cmake: zephyr modules variable names.

This commit refactors processing of Zephyr modules.
The Zephyr module name processing has been moved to zephyr_modules.cmake
to ensure that all `ZEPHYR_<MODULE_NAME>_MODULE_DIR` are defined early
in the build system.

No changes to inclusion of Zephyr module CMake code.

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