arduino-tftpboot fix

[ianmcc]

Hi, I encountered a problem installing the arduino-tftpboot bootloader on an EtherTen. The problem is a change in avrdude introduced around version 6.2, that changes the way extended fuse bytes are reported. This is described at https://github.com/arduino/Arduino/issues/5175

This leads to errors when trying to upload the bootloader such as:

avrdude: verification error, first mismatch at byte 0x0000
0xfd != 0x05

The solution is to modify hardware/arduino-tftpboot/boards.txt, the line

arduino-tftpboot.bootloader.extended_fuses=0x05

should be

arduino-tftpboot.bootloader.extended_fuses=0xfd

After this, it appears to upload successfully.