AP_HAL_Linux: rename macros to avoid conflicts

these macros were also defined in NuttX in clock.h

libraries/AP_HAL_Linux/Perf.cpp

@@ -41,7 +41,7 @@ static inline uint64_t now_nsec()
 {
     struct timespec ts;
     clock_gettime(CLOCK_MONOTONIC, &ts);
-    return ts.tv_nsec + (ts.tv_sec * NSEC_PER_SEC);
+    return ts.tv_nsec + (ts.tv_sec * AP_NSEC_PER_SEC);
 }
 
 Perf *Perf::get_instance()

libraries/AP_HAL_Linux/PollerThread.cpp

@@ -76,8 +76,8 @@ bool TimerPollable::adjust_timer(uint32_t timeout_usec)
 
     struct itimerspec spec = { };
 
-    spec.it_interval.tv_nsec = timeout_usec * NSEC_PER_USEC;
-    spec.it_value.tv_nsec = timeout_usec * NSEC_PER_USEC;
+    spec.it_interval.tv_nsec = timeout_usec * AP_NSEC_PER_USEC;
+    spec.it_value.tv_nsec = timeout_usec * AP_NSEC_PER_USEC;
 
     if (timerfd_settime(_fd, 0, &spec, nullptr) < 0) {
         return false;

libraries/AP_HAL_Linux/RCOutput_AeroIO.cpp

@@ -276,12 +276,12 @@ uint16_t RCOutput_AeroIO::_usec_to_hw(uint16_t freq, uint16_t usec)
 {
     float f = freq;
     float u = usec;
-    return 0xFFFF * u * f / USEC_PER_SEC;
+    return 0xFFFF * u * f / AP_USEC_PER_SEC;
 }
 
 uint16_t RCOutput_AeroIO::_hw_to_usec(uint16_t freq, uint16_t hw_val)
 {
     float p = hw_val;
     float f = freq;
-    return p * USEC_PER_SEC / (0xFFFF * f);
+    return p * AP_USEC_PER_SEC / (0xFFFF * f);
 }