POSIX: use px4_getpid()

The posix build only has one process so calling getpid() will not
provide the expected result.

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

src/platforms/posix/px4_layer/px4_posix_tasks.cpp

@@ -273,6 +273,21 @@ void px4_show_tasks()
 }
 
 __BEGIN_DECLS
+
+int px4_getpid()
+{
+	pthread_t pid = pthread_self();
+
+	// Get pthread ID from the opaque ID
+	for (int i=0; i<PX4_MAX_TASKS; ++i) {
+		if (taskmap[i].pid == pid) {
+			return i;
+		}
+	}
+	PX4_ERR("px4_getpid() called from non-thread context!");
+	return -EINVAL;
+}
+
 const char *getprogname();
 const char *getprogname()
 {