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

Nucleo F303RE: Add ADC DMA channels

Updated stm32f3discovery configuration

* Added notes about external debugging to README
* Added conditional pin assignments for I2C1 and I2C2 to prevent
  apps/system/i2c build failures.