cmake: Zephyr module error on module name current

In CMake, ZEPHYR_CURRENT_MODULE_DIR is used during processing of module
CMakeLists.txt files.
If a module is named `current` then it's corresponding
`ZEPHYR_<module_name>_MODULE_DIR` would become ZEPHYR_CURRENT_MODULE_DIR
and thus colliding with internal Zephyr CMake variable.

Using module name `current` is therefore forbidden.
Print an error to users when a module named `current` is used in
Zephyr.

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