mcu: Default the restart method to 'command' on non-serial ports

If the mcu supports command restarts and it does not appear to use a
real serial port, then default the restart method to 'command'.  This
is a better default on boards with native USB support.

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

config/example.cfg

@@ -264,7 +264,7 @@ serial: /dev/ttyACM0
 pin_map: arduino
 #   This option may be used to enable Arduino pin name aliases. The
 #   default is to not enable the aliases.
-#restart_method: arduino
+#restart_method:
 #   This controls the mechanism the host will use to reset the
 #   micro-controller. The choices are 'arduino', 'rpi_usb', and
 #   'command'. The 'arduino' method (toggle DTR) is common on Arduino
@@ -273,7 +273,8 @@ pin_map: arduino
 #   disables power to all USB ports to accomplish a micro-controller
 #   reset. The 'command' method involves sending a Klipper command to
 #   the micro-controller so that it can reset itself. The default is
-#   'arduino'.
+#   'arduino' if the micro-controller communicates over a serial port,
+#   'command' otherwise.
 
 # The printer section controls high level printer settings.
 [printer]

klippy/mcu.py

@@ -428,9 +428,9 @@ class MCU:
         # Restarts
         self._restart_method = 'command'
         if baud:
-            rmethods = {m: m for m in ['arduino', 'command', 'rpi_usb']}
+            rmethods = {m: m for m in [None, 'arduino', 'command', 'rpi_usb']}
             self._restart_method = config.getchoice(
-                'restart_method', rmethods, 'arduino')
+                'restart_method', rmethods, None)
         self._reset_cmd = self._config_reset_cmd = None
         self._emergency_stop_cmd = None
         self._is_shutdown = self._is_timeout = False
@@ -600,6 +600,12 @@ class MCU:
         self._emergency_stop_cmd = self.lookup_command("emergency_stop")
         self._reset_cmd = self.try_lookup_command("reset")
         self._config_reset_cmd = self.try_lookup_command("config_reset")
+        if (self._restart_method is None
+            and (self._reset_cmd is not None
+                 or self.config_reset_cmd is not None)
+            and self._serial.msgparser.get_constant(
+                'SERIAL_BAUD', None) is None):
+            self._restart_method = 'command'
         self.register_msg(self.handle_shutdown, 'shutdown')
         self.register_msg(self.handle_shutdown, 'is_shutdown')
         self.register_msg(self.handle_mcu_stats, 'stats')