Add Atmel JTAGICE3 to externalprogrammers.txt
Add Atmel JTAGICE3 to externalprogrammers.txt

- Atmel JTAGICE3 (ISP mode)
- Atmel JTAGICE3 (JTAG mode)

MCUdude/MajorCore,MCUdude/MajorCore,MCUdude/MajorCore