boards: nrf5340: modify the hex file when flashing the merged binary

When instructed to flash the combined Secure and Non-Secure
binary, we need to modify the hex file used in west flash.
The combined binary is named tfm_merged.hex, regardless of
building with or without BL2.

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