cmake: ZEPHYR_MODULES_NAMES list created before ext-root
cmake: ZEPHYR_MODULES_NAMES list created before ext-root

Create the ZEPHYR_MODULES_NAMES list of zephyr modules present in the
system before calling roots defined in `MODULE_EXT_ROOT`.

This will allow external module roots to process based on Zephyr modules
available in the system.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr