Kicking the rescheduling procedure (xnsched_run()) from call sites
running over the root thread with hard irqs enabled may lead to
missing a rescheduling opportunity as a consequence of a CPU
migration, e.g.:

    CPU0					CPU1
    ----                         		----
    t0: ...
            <context switch>
    t1: xnthread_resume(t0)
             xnsched_run()
             <CPU MIGRATION -->
                  |                             |
                  |                             |
                  x                             |
          (missed rescheduling)                 v
                                    ___xnsched_run(CPU1.sched)

To address this issue, make sure every call to xnsched_run() either
runs from the head domain which is not affected by CPU migration, or
moves into the latest atomic section which might have changed the
scheduler state.

Turning on CONFIG_XENO_OPT_DEBUG_COBALT also enables a (oneshot)
assertion which detects invalid calls from the root domain with hard
irqs enabled.
Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>