boards:mega2560: add debug and debug-server targets

This PR adds the make tagets `debug` and `debug-server` to the Arduino Mega 2560.

Tested in conjunction with the AVR Dragon board as ISP/OCD tool.
Kind of.

It might be unusual but I've used similar exports sometimes to save having to set up the build environment. Either way I don't see any benefit to unconditionally setting it in this case.