Fix _listen_until_online to have the desired behavior

Before you sometimes had to connect twice to actually connect because the M105
was sent before the printer's Serial module actually became ready. Now when no
data is available to read on the Serial line a new M105 is sent until
connection is established.

printcore.py

@@ -138,7 +138,7 @@ class printcore():
             self._send("M105")
             while self._listen_can_continue():
                 line = self._readline()
-                if line == None:
+                if not line:
                     break
                 if line.startswith(tuple(self.greetings)) or line.startswith('ok'):
                     if self.onlinecb: