cmake: tf-m: create tfm_api dependency on tfm_s.hex

Incremental builds for TF-M are not picked up by Zephyr linking stage.
Code changes to tf-m repository results in a rebuild of TF-M and thus
an updated tfm_s.hex (and other files).

tfm_s.hex is merged together with the zephyr hex to form a final merged
hex file for flashing. This is done as a post-build command, however
such as step cannot take extra dependencies. The Zephyr target can have
extra dependencies, however that will only ensure the dependency is
brought up-to-date when Zephyr re-link, not re-linking Zephyr when the
dependency changes.

Therefore an object dependency is placed on the interface.c file for
Zephyr TF-M interface implementation, which ensures the tfm_api library
is brought up-to-date whenever TF-M rebuilds, and this update again
ensures the Zephyr itself is re-linked whenever TF-M rebuilds.

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