x86: Init the debug UART if enabled

If the debug UART is enabled, get it ready for use at the earliest possible
opportunity. This is not actually very early, but until we have a stack it
is difficult to make it work.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/start.S

@@ -126,14 +126,9 @@ car_init_ret:
 	call	board_init_f_mem
 	mov	%eax, %esp
 
-	/*
-	 * Debug UART is available here although it may not be plumbed out
-	 * to pins depending on the board. To use it:
-	 *
-	 * call  debug_uart_init
-	 * mov   $'a', %eax
-	 * call  printch
-	 */
+#ifdef CONFIG_DEBUG_UART
+	call	debug_uart_init
+#endif
 
 	/* Get address of global_data */
 	mov	%fs:0, %edx