Added "mighty_twox-2560" for Rep2X's with Atmega2560's

1.  Updated SConstruct to allow 'mighty_twox-2560' target.

2.  Updated SConscript.mightyboard.
    - duplicated mighty_twox target, and removed code squeezing (since code squeezing is not apparently used on 2560 targets.
    - added override for 2560 targets to use stk500v2 protocol for avrdude

3.  Updated supporting scripts (build.sh, checksize.sh) to allow mighty_twox-2560 target