Nucleo

Hello,

This adds a first support for the NUCLEO F103RB board:
* [Official presentation](http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/LN1847/PF259875?icmp=nucleo-ipf_pron_pr-nucleo_feb2014&sc=nucleoF103RB-pr)
* [Product on mouser](http://www.mouser.fr/ProductDetail/STMicroelectronics/NUCLEO-F103RB/?qs=fK8dlpkaUMs2W6dqxUbL2w%3D%3D)

The code have been tested on an actual board, blinking the led and using Serial2 to communicate via the ST-LINK works

Mainly, these commits adds:
* The `nucleo` board
* The generation of a `.hex` in the `build/` directory, which is required in order to flash the board via JTAG & OpenOCD
* The `none` bootloader, that write everything at `0x08000000` (Nucleo features a JTAG, so it's nicer to place BOOTLOADER to `none` that typing `make jtag` everytime)
* The option `HAS_EXTERNAL_OSC` in the Makefile (default to yes) that can disable the external oscillator and use the internal one (running the board at 36mhz). This is because the Nucleo doesn't have an external oscillator (you can solder one, though)
These are weak symbols -- you can just override them in your board file.