stm32l: fix new line output on the UART

On the STM32L UART without a FIFO, the Transmit Register might need
some time before pushing its content to the shift register.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=None
TEST=on ADV board, verify we are no longer missing most of the carriage
return on console.

Change-Id: Ic638dc452d6e30a5f127710fc964143d477fa1d6

chip/stm32l/uart.c

@@ -59,6 +59,11 @@ int uart_rx_available(void)
 
 void uart_write_char(char c)
 {
+	/* we normally never wait here since uart_write_char is normally called
+	 * when the buffer is ready, excepted when we insert a carriage return
+	 * before a line feed in the interrupt routine.
+	 */
+	while (!uart_tx_ready()) ;
 	STM32L_USART_DR(UARTN) = c;
 }