Squashed commit of the following:

    sched/:  Convert legitimate uses of task_create() to nxtask_create().  Review handling of returned values from all uses of kthread_create() (as well as nxtask_create()).
    graphics/:  Review return values for all calls to kthread_start() because it no longer returns an errno.
    drivers/:  threads started by drivers should be kernel threads, not user tasks.  Review return values for all calls to kthread_start() because it no longer returns an errno.
    configs/:  threads started by board bringup logic should be kernel threads, not user tasksi (part 2 of 2).
    sched/task:  Add nxtask_create().  Kthread_create() and nxtask_create() are internal OS functions and should not modify the errno variable.  configs/:  threads started by board bringup logic should be kernel threads, not user tasks.