build support and limited GPIO mappings for atmega640/1280/2560
add mega2560 PortE-PortL + UART0/1, Timer3
further mega2560 gpio changes
add mega2560 serial ports #2 and #3
temporary makefile changes to support atmega2560
greater configurability in build process
Merge pull request #3 from indigoid/atmega2560-basics

Atmega2560 basics