Add Polled Mode Serial Output for Panic Messages

[genosensor]

I propose adding a polled output mode so panic messages can be output on a serial port with interrupts disabled.
This gives some feedback as to the cause of a crash in situations where no debugging probe is installed.
Here's a hacked up example for the STM32v1 serial drivers:

Code: Select all

/**
 * @brief   Switch to polling mode (disable interrupts)
 *
 * @param[in] sdp       communication channel associated to the USART
 */
void sd_lld_pollMode(SerialDriver *sdp) {
  USART_TypeDef *u = sdp->usart;
  u->CR1 &=
    ~(USART_CR1_TXEIE | USART_CR1_TCIE | USART_CR1_RXNEIE | USART_CR1_PEIE);
  u->CR2 &= ~USART_CR2_LBDIE;
  u->CR3 &= ~USART_CR3_EIE;
}


/**
 * @brief   Output byte in polling mode
 *
 * @param[in] sdp       communication channel associated to the USART
 */
void sd_lld_xmitByte(SerialDriver *sdp, uint8_t b) {
  USART_TypeDef *u = sdp->usart;
  while (!(u->SR & USART_SR_TXE)) ;
  u->DR = b;
}