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.
Hm, maybe an example:
I have two devices to flash - node and gateway.
Now, I set PROGRAMMEFLAGS to use device A for the gateway at all times and to use device B for the node at all times. That way, I can have both devices attached at the same time and make sure the correct application is deployed.

This is probably not a Makefile that will be part of RIOT, but very handy for me as a developer.