serial: Call flush() before changing baudrate

Changing baudrate is a sensitive operation. To ensure that U-Boot messages
printed before changing baudrate are not lost, call new U-Boot console
flush() function.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial-uclass.c

@@ -476,6 +476,7 @@ static int on_baudrate(const char *name, const char *value, enum env_op op,
 			printf("## Switch baudrate to %d bps and press ENTER ...\n",
 			       baudrate);
 			udelay(50000);
+			flush();
 		}
 
 		gd->baudrate = baudrate;