feat(serial): Add function to switch serial to synchronous

In severe error conditions, asynchronous prints will never work.  For
such cases we need a way to make prints happen synchronously again, the
same way it works during early boot.  Add a serial_return_to_synchronous()
function which unconditionally switches the serial driver code to
synchronous mode.

Only use this function in unrecoverable error conditions!

Signed-off-by: Rahix <rahix@rahix.de>

epicardium/modules/modules.h

@@ -31,6 +31,8 @@ void vSerialTask(void *pvParameters);
 void serial_enqueue_char(char chr);
 void serial_flush(void);
 extern TaskHandle_t serial_task_id;
+/* Turn off the print queue and do prints synchroneous from now on. */
+void serial_return_to_synchronous();
 
 // For the eSetBit xTaskNotify task semaphore trigger
 enum serial_notify{

epicardium/modules/serial.c

@@ -46,6 +46,11 @@ void serial_init()
 	);
 }
 
+void serial_return_to_synchronous()
+{
+	write_stream_buffer = NULL;
+}
+
 /*
  * API-call to write a string.  Output goes to both CDCACM and UART
  */