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.
What is the benefit of using `?=` ?
I'm just asking because it works for me this way. The `-b 115200` can be moved into `FFLAGS` but it doesn't seem to have any effect when using the Dragon. `-P $(PORT)` can't be moved into the FFLAGS because it breaks flashing for the Dragon (the device at `$(PORT)` is the Arduino, not the Dragon (which as far as I can tell doesn't get a device assigned (apart from the default under `/dev/bus/usb/...`. Therefore it would require another kind of override to but it into FFLAGS.