Do not check flash button during calibration

It does not make sense to toggle night mode
or start another round of countdown during
calibration.

Closes #4.

ampel-firmware/ampel-firmware.ino

@@ -141,7 +141,12 @@ void loop() {
   keepServicesAlive();
 
   // Short press for night mode, Long press for calibration.
-  checkFlashButton();
+  // Inactive during calibration.
+  if (!(sensor::current_state == sensor::PREPARE_CALIBRATION_STABLE ||
+        sensor::current_state == sensor::PREPARE_CALIBRATION_UNSTABLE))
+  {
+    checkFlashButton();
+  }
 
   checkSerialInput();