syscall: clock_systimer() is no longer a system call. It has been replaced with the equivalent, standard interface clock() as the system call.

sched/clock:  Move the implementation of clock() from libs/libc/time to sched/clock.  This is necessary because it calls the (now) internal OS function clock_systimer.  clock() is now accessed only via a system call in certain configuratins.
libs/libc/wqueue:  Replace calls to clock_systimer() with calls to the equivalent clock().

libs/libc/time/Make.defs

@@ -38,7 +38,7 @@
 
 CSRCS += lib_strftime.c lib_calendar2utc.c lib_daysbeforemonth.c
 CSRCS += lib_gettimeofday.c lib_isleapyear.c lib_settimeofday.c lib_time.c
-CSRCS += lib_difftime.c lib_clock.c
+CSRCS += lib_difftime.c
 
 ifndef CONFIG_DISABLE_SIGNALS
 CSRCS += lib_nanosleep.c

libs/libc/time/lib_clock.c

@@ -1,74 +0,0 @@
-/****************************************************************************
- * libs/libc/time/lib_clock.c
- *
- *   Copyright (C) 2015 Gregory Nutt. All rights reserved.
- *   Author: Gregory Nutt <gnutt@nuttx.org>
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- *
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in
- *    the documentation and/or other materials provided with the
- *    distribution.
- * 3. Neither the name NuttX nor the names of its contributors may be
- *    used to endorse or promote products derived from this software
- *    without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
- * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
- * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
- * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
- * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
- * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
- * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
- * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
- * POSSIBILITY OF SUCH DAMAGE.
- *
- ****************************************************************************/
-
-/****************************************************************************
- * Included Files
- ****************************************************************************/
-
-#include <nuttx/config.h>
-
-#include <time.h>
-
-#include <nuttx/clock.h>
-
-/****************************************************************************
- * Public Functions
- ****************************************************************************/
-
-/****************************************************************************
- * Name:  clock
- *
- * Description:
- *   The clock() function returns the implementation's best approximation to
- *   the processor time used by the process since the beginning of a
- *   implementation-defined era related only to the process invocation.
- *
- *   To determine the time in seconds, the value returned by clock() should
- *   be divided by the value of the macro CLOCKS_PER_SEC as defined in <time.h>.
- *
- * Input Parameters:
- *   None
- *
- * Returned Value:
- *   The system time in units of clock ticks is returend.  If the processor
- *   time used is not available or its value cannot be represented, the
- *   function will return the value (clock_t)-1.
- *
- ****************************************************************************/
-
-clock_t clock(void)
-{
-  return (clock_t)clock_systimer();
-}

libs/libc/wqueue/work_queue.c

@@ -114,7 +114,7 @@ static int work_qqueue(FAR struct usr_wqueue_s *wqueue,
 
   /* Now, time-tag that entry and put it in the work queue. */
 
-  work->qtime  = clock_systimer(); /* Time work queued */
+  work->qtime  = clock(); /* Time work queued */
 
   dq_addlast((FAR dq_entry_t *)work, &wqueue->q);
   kill(wqueue->pid, SIGWORK);   /* Wake up the worker thread */

libs/libc/wqueue/work_usrthread.c

@@ -141,7 +141,7 @@ void work_process(FAR struct usr_wqueue_s *wqueue)
 
   /* Get the time that we started this polling cycle in clock ticks. */
 
-  stick = clock_systimer();
+  stick = clock();
 
   /* And check each entry in the work queue.  Since we have locked the
    * work queue we know:  (1) we will not be suspended unless we do
@@ -157,7 +157,7 @@ void work_process(FAR struct usr_wqueue_s *wqueue)
        * zero.  Therefore a delay of zero will always execute immediately.
        */
 
-      ctick   = clock_systimer();
+      ctick   = clock();
       elapsed = ctick - work->qtime;
       if (elapsed >= work->delay)
         {
@@ -251,7 +251,7 @@ void work_process(FAR struct usr_wqueue_s *wqueue)
 
   /* Get the delay (in clock ticks) since we started the sampling */
 
-  elapsed = clock_systimer() - stick;
+  elapsed = clock() - stick;
   if (elapsed < wqueue->delay && next > 0)
     {
       /* How must time would we need to delay to get to the end of the