1. Implement the data extraction function for XBEE_msg if it's a Receive packet XBEE module has received from RF.
2. Fix some bugs in parsing the serial data from the XBEE module
3. Able to identify the source serial number of the received RF packet.
TODO:
1. The checksum algorithm
2. Possible bugs when identifying the serial number of the source of the received RF packet (seen once in gdb debuging, but no reapperance)