Adding support for NUCLEO-F103RB

This adds support for the NUCLEO-F103RB boards, using changes introduced by @mbolivar allowing boards to override clocks (see #102)

Changes are:
* It adds `.hex` output in the `build/` directory, that is compliant with `openocd`
* It adds comments in the `Makefile` to allow overriding of the `STM32_PCLK*` using CFLAGS
* It adds `nucleo` board support, overriding clocks to use the internal oscillator

Note that since there is no bootloader, you'll also have to run `make jtag` to build a valid image for the board.