Squashed commit of the following:

sched/init/nx_bringup.c:  Fix a naming collision.
    sched/init:  Rename os_start() to nx_start()
    sched/init:  Rename os_smp* to nx_smp*
    sched/init:  Rename os_bringup to nx_bringup
    sched/init:  rename all internal static functions to begin with nx_ vs os_

arch/xtensa/src/common/xtensa_idle.c

@@ -75,7 +75,7 @@ void up_idle(void)
    */
 
   /* This is a kludge that I still don't understand.  The call to kmm_trysemaphore()
-   * in the os_start.c IDLE loop seems necessary for the good health of the IDLE
+   * in the nx_start.c IDLE loop seems necessary for the good health of the IDLE
    * loop.  When the work queue is enabled, this logic is removed from the IDLE
    * loop and it appears that we are somehow left idling with interrupts non-
    * functional. The following should be no-op, it just disables then re-enables

arch/xtensa/src/esp32/esp32_start.c

@@ -152,6 +152,6 @@ void IRAM_ATTR __start(void)
 
   /* Bring up NuttX */
 
-  os_start();
+  nx_start();
   for(; ; ); /* Should not return */
 }