automation: Explicitly enable NULL scheduler for boot-cpupools test

automation: Explicitly enable NULL scheduler for boot-cpupools test

NULL scheduler is not enabled by default on non-debug Xen builds. This
causes the boot time cpupools test to fail on such build jobs. Fix the issue
by explicitly specifying the config options required to enable the NULL
scheduler.

Fixes: 36e3f4158778 ("automation: Add a new job for testing boot time cpupools on arm64")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>