All AVR toolchain configuration names must end with _TOOLCHAIN to work properly with nuttx/tools/testbuilds.sh

ATMEGA2560: Introduce ostest variant for Arduino Mega2560

sim/nsh: hello task sometimes needs a larger stack size or things get clobbered when calling into the system write() interface

Arduino-Atmega2560: Updates from review of merge plus name changes to be consistent with the new conventions that I am trying to establish

Arduino Mega 2560: Introduce 'Hello World' example