Prevent interruption of TouchUI calibration screen

- Keep LCD messages from interrupting touch calibration wizard.
- Avoid corrupting the display list when this happens.

Marlin/src/lcd/extui/lib/ftdi_eve_touch_ui/screens/status_screen.cpp

@@ -301,6 +301,14 @@ void StatusScreen::setStatusMessage(progmem_str message) {
 }
 
 void StatusScreen::setStatusMessage(const char* message) {
+  if(CommandProcessor::is_processing()) {
+      #if ENABLED(TOUCH_UI_DEBUG)
+        SERIAL_ECHO_START();
+        SERIAL_ECHOLNPGM("Cannot update status message, command processor busy");
+      #endif
+      return;
+  }
+
   CommandProcessor cmd;
   cmd.cmd(CMD_DLSTART)
      .cmd(CLEAR_COLOR_RGB(Theme::bg_color))

Marlin/src/lcd/extui/lib/ftdi_eve_touch_ui/screens/touch_calibration_screen.cpp

@@ -85,6 +85,9 @@ void TouchCalibrationScreen::onRedraw(draw_mode_t) {
 void TouchCalibrationScreen::onIdle() {
   if (!CLCD::is_touching() && !CommandProcessor::is_processing()) {
     GOTO_PREVIOUS();
+    #if ENABLED(TOUCH_UI_DEBUG)
+      SERIAL_ECHO_MSG("Calibration routine finished");
+    #endif
   }
 }