Replacing usleep with px4_usleep

This is a step towards isolating time from the system.

src/lib/cdev/posix/cdev_platform.cpp

@@ -340,7 +340,7 @@ extern "C" {
 #endif
 
 		while (sim_delay) {
-			usleep(100);
+			px4_usleep(100);
 		}
 
 		PX4_DEBUG("Called px4_poll timeout = %d", timeout);

src/lib/cdev/test/cdevtest_example.cpp

@@ -234,7 +234,7 @@ int CDevExample::do_poll(int fd, int timeout, int iterations, int delayms_after_
 		}
 
 		if (delayms_after_poll) {
-			usleep(delayms_after_poll * 1000);
+			px4_usleep(delayms_after_poll * 1000);
 		}
 
 		loop_count++;