[stm32-ocm3] Use revised paths and defines
[stm32-ocm3] Get serial input working properly

The original code here was largely "standard" newlib syscalls that
expected to be connected to stdio (scanf and friends)  However Contiki
manages input itself, via calling the serial_line_input_byte() from your
rx ISR.

Along the way a bug in overflowing the output buffer was fixed.