cmake: zephyr_module.cmake ZEPHYR_EXTRA_MODULES evaluated too late

Previously, `cmake/app/boilerplate.cmake` evaluated
ZEPHYR_EXTRA_MODULES on line ~548 *after* `zephyr_module.cmake`
was included on line ~168.

This change ensures that modules defined in `ENV{ZEPHYR_EXTRA_MODULES}`
are processed properly through `zephyr_module.cmake`.

Fixes #30673

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>