modules: tf-m: fix NS image selection for regression tests

When running with TFM Regression option set, but without
BL2 (MCUboot) we need to use the TF-M's NS binary, and not
the Zephyr application. This was selected properly for the
case with MCUboot boot already, but not when running the
regression tests without MCUboot.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>