Port to stm32-p103 for qemu simulation

It has been tested on qemu only, so we need someone to test it on a real board.
Though I have split board- and platform- specific files for sanity,
there is not much difference between p103 and discoveryf4.
I've modified necessary parts only, and found that most of the code remains duplicate, 
so I think the hierarchy should be considered again.
Maybe it's better to have stm32f4x and stm32f10x merged as one single target.

Btw, I found it works without changing compile option from `-mcpu=cortex-m4` to `-mcpu=cortex-m3`.

### Build Instructions
 * Build the qemu fork which supports p103 (https://github.com/konyavic/qemu_stm32) (with a trivial bugfix)
 * Prefix each make command with BOARD=p103, e.g. `BOARD=p103 make config`
 * Configuration
  * Turn off ELF loader and bit-band region
  * Change debug serial IO to USART2
 * Run the command bellow:
```
<path_qemu_stm32>/arm-softmmu/qemu-system-arm -M stm32-p103 -kernel <path_f9>/build/p103/f9.bin -serial stdio
```