Adds support for 3 Cortex-M boards, selectable via "BOARD" in the Makefile:
- microbit, Cortex-M0 via nRF51822
- netduino2, Cortex-M3 via STM32F205
- mps2-an385, Cortex-M3 via FPGA
netduino2 is the default board because it's supported by older qemu
versions (down to at least 2.5.0).
Damien George