set correct max size for mega2560 (to address issue #2277)

This should address issue mentioned here
https://github.com/arduino/Arduino/issues/2277

According to available docs and datasheets, the issue raised is valid, and the max size mentioned on boards.txt is 4Kib more than it should have been.

Signed-off-by: Arnav Gupta <championswimmer@gmail.com>