cpu: make stm32f{0,1,3,4} comply to UART0 module

This adds ability to use UART0 module for the new CPUs too. Alternative to https://github.com/RIOT-OS/RIOT/pull/1465 to keep the examples clean.

DISCLAIMER:
Only tested for stm32f1 (which is not part of this) due to lack of hardware.
we should enable getchar to work with uart0:
```c
    char *res = (char*)buffer;
    res[0] = (char)uart0_readc();
    return 1;
```