gitlab-ci: add an ARM32 qemu-based smoke test

gitlab-ci: add an ARM32 qemu-based smoke test

Add a minimal ARM32 smoke test based on qemu-system-arm, as provided by
the test-artifacts qemu container. The minimal test simply boots Xen
(built from previous build stages) and Dom0.

The test needs a working kernel and minimal initrd for dom0. Instead of
building our own kernel and initrd, which would mean maintaining one or
two more builting scripts under automation/, we borrow a kernel and
initrd from distros.

For the kernel we pick the Debian Bullseye kernel, which has everything
we need already built-in. However, we cannot use the Debian Bullseye
initrd because it is 22MB and the large size causes QEMU to core dump.

Instead, use the tiny busybox-based rootfs provided by Alpine Linux,
which is really minimal: just 2.5MB. Note that we cannot use the Alpine
Linux kernel because that doesn't boot on Xen.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>