boards: nrf5340: flash by default the combined TF-M + Zephyr binary

In nRF5340, instruct the build to flash, by default, the
combined Secure (TF-M) and Non-Secure (Zephyr) binaries
as a merged binary, using west flash, if we are building
in-tree tests.

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