tf-m: Fix flashing of samples on nrf5340 and nrf9160 boards

Always flash the merged hex file.

This fixes flashing for samples. Before "west flash" only worked for
tests.

I don't know why this was not done from the get-go.

This also fixes builds that enables CONFIG_TFM_REGRESSION_NS, which will
use the tfm_ns application file instead of the zephyr application, and
will merge tfm_ns hex into the merged hex file. Otherwise the wrong
application hex file will be flashed.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>