QuRT: Changes to enable qurt target to build

QuRT doesn't support unlink and does not provide getpid().
The DSPAL layer provides access to usleep so an implementation is
no longer needed.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/uORB/uORBDevices_posix.cpp

@@ -91,7 +91,7 @@ uORB::DeviceNode::open(device::file_t *filp)
     lock();
 
     if (_publisher == 0) {
-      _publisher = getpid();
+      _publisher = px4_getpid();
       ret = PX4_OK;
 
     } else {
@@ -153,7 +153,7 @@ uORB::DeviceNode::close(device::file_t *filp)
 {
   //warnx("uORB::DeviceNode::close fd = %d", filp->fd);
   /* is this the publisher closing? */
-  if (getpid() == _publisher) {
+  if (px4_getpid() == _publisher) {
     _publisher = 0;
 
   } else {