Add open function without path

[sebromero]

Currently, when creating a file with createFile the file is already open. However if we close the file and later want to re-open it, we'd need to provide the full file path which might be unknown at that moment. e.g.

logFile = storage.getRootFolder().createFile("log.txt", FileMode::WRITE);
logFile.close();        
...
logFile.open("/usb/log.txt", FileMode::APPEND);
logFile.write(message);

This PR adds a function that re-uses the path set by createFile and getRootFolder respectively.