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 <Torsten.Rasmussen@nordicsemi.no>