Atmega2560 basics

Struggling a bit with git, but I think this is right now

- add remaining ATmega640/1280/2560 GPIO Ports E-L
- add ATmega640/1280/2560 Serial2 and Serial3
- some makefile hackery to eliminate the need to edit the makefile in many cases

Missing: ATmega640/1280/2560 additional PWM pin definitions for GPIO Ports E-L