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.
Since PORT can be (and has to be) overridden anyways if two different boards should be flashed via one Makefile I think, this shouldn't be necessary. I can change it anyways though, if you consider it better...