Ported to STM32F401 Nucleo board.

The Nucleo boards are like the Discovery boards in that they come with an
attached STLINK-V2 programmer. The MCU's UART2 is routed through the
programmer's USB connection and it appears as a VCP device on the host.

So far, this has only been tested to the extent that the elua shell and
lua prompt appear to be functioning as expected.
Merge pull request #44 from smartavionics/stm32f4-nucleo

Ported to STM32F401 Nucleo board.