orb: add a separate uORB::Manager::initialize() method

This fixes a race condition: uORB::Manager::get_instance() is used in a
multi-thread context, but the singleton initialization was not thread-safe.
Further, this avoids having to check for nullptr every time the singleton
is accessed.

uORB::Manager::initialize() is called when uorb is started. No one else
accesses the singleton before that point, because it is only used in the
orb_* methods, and in muorb. Both require uorb to be started already when
they are used.

src/modules/uORB/uORBManager.cpp

@@ -49,13 +49,13 @@ uORB::Manager *uORB::Manager::_Instance = nullptr;
 
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
-uORB::Manager *uORB::Manager::get_instance()
+bool uORB::Manager::initialize()
 {
 	if (_Instance == nullptr) {
 		_Instance = new uORB::Manager();
 	}
 
-	return _Instance;
+	return _Instance != nullptr;
 }
 
 //-----------------------------------------------------------------------------