boards: Run the combined an547 hex in qemu

Zephyr will try to run the `zephyr.elf` file by default. This is
problematic for any `..._ns` target, as they all need to boot through
[mcuboot and] TF-M. They simply hardfault without the rest of their
system image.

This corrects the `west build -t run ...` for the `mps3_an547_ns`
board, as it now has special treatment.

Signed-off-by: Jimmy Brisson <jimmy.brisson@linaro.org>