firmware/shim: UNSUPPORTED=n

firmware/shim: UNSUPPORTED=n

We shouldn't default to include any unsupported code in the shim. Mark
the setting as off, replacing the ARGO specification. This points out
anomalies with the scheduler configuration: Unsupported schedulers
better don't default to Y in release builds (like is already the case
for ARINC653). Without at least the SCHED_NULL adjustments, the shim
would suddenly build with RTDS as its default scheduler.

As a result, the SCHED_NULL setting can also be dropped from defconfig.

Clearly with the shim defaulting to it, SCHED_NULL must be supported at
least there.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>