serialhdl: Make dump_debug() output atomic

Build a single (very large) logging message with the debug state.
This prevents the output from being fragmented.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/mcu.py

@@ -451,8 +451,8 @@ class MCU:
             return
         self._is_shutdown = True
         self._shutdown_msg = msg = params['#msg']
-        logging.info("%s: %s", params['#name'], self._shutdown_msg)
-        self._serial.dump_debug()
+        logging.info("MCU '%s' %s: %s\n%s", self._name, params['#name'],
+                     self._shutdown_msg, self._serial.dump_debug())
         prefix = "MCU '%s' shutdown: " % (self._name,)
         if params['#name'] == 'is_shutdown':
             prefix = "Previous MCU '%s' shutdown: " % (self._name,)