modules: tf-m: introduce option to configure flash binary

Introduce an option to instruct the build system
to flash the Non-Secure (Zephyr) firmware image
together with the TF-M (Secure) firmware image
as a single merged binary, instead of just flashing
the Non-Secure application.

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