qemu: Try to automatically boot from the QEMU firmware device (qfw)

At the moment the QEMU boot sequence tries various (storage) devices
when trying to find a payload to boot.
To simplify starting a specific kernel and initrd, there is also the qfw
command, which can use the files specified on the QEMU command line, via
the -kernel and -initrd options.
Add this command to the list of boot options to try. Since users
specifying those options on the command line probably explicitly want
to run them, let's place the new command first. Without those options,
the qfw command will just gracefully fail, and we continue with the
existing order.

This allows auto-booting of specific kernels in QEMU, for instance in CI
systems.

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