cmake: warn the user if a Zephyr library is created in app-mode

Fixes: #19582

When `find_package(Zephyr)` completes then all boilerplate code has been
processed and all Zephyr libraries has been placed inside the
whole archive flags.

Also all libs dependencies has been processed.

This is indicated by the presence of the zephyr_prebuilt target.

Thus, warn the user if `zephyr_library()` / `zephyr_library_named()`
is called in app-mode.

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