feat(qemu): combine TF-A artefacts into ROM file

At the moment the QEMU platform documentation asks users to combine
bl1.bin and fip.bin in a certain way to get an image that can be used on
the QEMU command line.

Let the build system actually create those files already, if we provide
a non-secure payload (BL33), to simplify deployment and spare the user of
fragile operations.

Change-Id: Ia35f17b9d60aab41f0d53f1f73a1326a316c4950
Signed-off-by: Andre Przywara <andre.przywara@arm.com>