Rover: changed SERIAL3_BAUD to SERIAL1_BAUD

and add SERIAL2_BAUD for systems that have it
Plane: added SERIAL2_BAUD and rename SERIAL3_BAUD to SERIAL1_BAUD
AP_HAL: added get_system_id() method on Util
HAL_PX4: support get_system_id() on PX4
Plane: use log system ID if available
Rover: log system ID if available
Copter: log system ID if available