AP_HAL_Linux: add method to check caller same as thread

libraries/AP_HAL_Linux/Thread.cpp

@@ -91,4 +91,9 @@ bool Thread::start(const char *name, int policy, int prio)
     return true;
 }
 
+bool Thread::is_current_thread()
+{
+    return pthread_equal(pthread_self(), _ctx);
+}
+
 }

libraries/AP_HAL_Linux/Thread.h

@@ -40,6 +40,8 @@ public:
 
     bool start(const char *name, int policy, int prio);
 
+    bool is_current_thread();
+
 protected:
     static void *_run_trampoline(void *arg);