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.