stm32f0: be UART0 aware
stm32f4: be UART0 aware
stm32f3: be UART0 aware
stm32f1: unify UART0 use
stm32f*: rename STDIO buffer and enable getchar with UART0
Merge pull request #1471 from thomaseichinger/impl_uart0

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