bug fixed and code improvements

Fixed a bug where the default binding IP for the TFTP server is set to
ip=‘’. By setting the default binding IP to ip=‘0.0.0.0’ then the TFTP
server is accessible by all IPs on the machine.

Improved code in server.py by replacing while True with .isAlive()
statements for the individual threads so that the while loop stays
alive while the threads are alive rather than just an arbitrary ‘True’