stm32f4xx: configurable usart module

[forGGe]

Current implementation of the usart_bus platform module:

    // Configure UART
    // TODO: make configuration values be chosen at compile time
    init_struct.USART_BaudRate             = 115200;
    init_struct.USART_WordLength           = USART_WordLength_8b;
    init_struct.USART_StopBits             = USART_StopBits_1;
    init_struct.USART_Parity               = USART_Parity_No;
    init_struct.USART_Mode                 = USART_Mode_Rx | USART_Mode_Tx;
    init_struct.USART_HardwareFlowControl  = USART_HardwareFlowControl_None;

It contains hard-coded init values. The issue is about creating compile-time configuration of the particular usart bus.

[forGGe]

Merged.