Rover: set GPS non-blocking

the new GPS driver only ever needs a non-blocking port

APMrover2/system.pde

@@ -262,6 +262,7 @@ static void startup_ground(void)
     mission.init();
 
     hal.uartA->set_blocking_writes(false);
+    hal.uartB->set_blocking_writes(false);
     hal.uartC->set_blocking_writes(false);
 
 	gcs_send_text_P(SEVERITY_LOW,PSTR("\n\n Ready to drive."));