Merge code from others more updated projects

[q2dg]

If you want to finish this bootloader (as you say in Readme), this suggestion could be save you many efforts. I've just found some other tftp bootladers very interesting that may interest you too:
https://github.com/codebendercc/Ariadne-Bootloader
https://github.com/per1234/Ariadne-Bootloader
https://github.com/freetronics/arduino-tftpboot