automation: add dom0less to the QEMU aarch64 smoke test

automation: add dom0less to the QEMU aarch64 smoke test

Add a trivial dom0less test:
- fetch the Debian arm64 kernel and use it ad dom0/U kernel
- use busybox-static to create a trivial dom0/U ramdisk
- use ImageBuilder to generate the uboot boot script automatically
- install and use u-boot from the Debian package to start the test
- binaries are loaded from uboot via tftp

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Wei Liu <wl@xen.org>