automation: Use custom build jobs when extra config options are needed

automation: Use custom build jobs when extra config options are needed

Currently, all the arm64 defconfig build jobs, regardless of the
container used, end up building Xen with the extra config options
specified in the main build script (e.g. CONFIG_EXPERT,
CONFIG_STATIC_MEMORY). Because these options are only needed for
specific test jobs, the current behavior of the CI is incorrect
as we add the extra options to all the defconfig builds. This means
that on arm64 there is not a single job performing proper defconfig build.

To fix this issue, add custom build jobs each time there is a need for
building Xen with additional config options. Introduce EXTRA_XEN_CONFIG
variable to be used by these jobs to store the required options. This
variable will be then read by the main build script to modify the .config
file. This will also help users to understand what is needed to run specific
test.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>