cmake: align Zephyr module variable to EXTRA_ZEPHYR_MODULES

Align Zephyr modules with other user facing variables where settings
can be defined or extended, meaning Zephyr modules now supports:
ZEPHYR_MODULES and EXTRA_ZEPHYR_MODULES.

Support for ZEPHYR_EXTRA_MODULES is kept foir backward compatibility.

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