Add a bunch of stuff: Add baudrate reconfig command to shell and move the uart to separate C file

stm-firmware/stm-periph/uart.c

@@ -95,6 +95,14 @@ void uart_change_brr(struct stm_uart *uart, uint32_t brr)
 	uart->uart_dev->BRR = brr;
 }
 
+uint32_t uart_get_brr(struct stm_uart *uart)
+{
+	if (!uart || !uart->uart_dev)
+		return 0;
+
+	return uart->brr_val;
+}
+
 void uart_disable(struct stm_uart *uart)
 {
 	if (!uart)