co2_sensor: Make co2 alert threshold configurable

Previously, it was hardcoded to 2000 ppm, which
remains the default value.

ampel-firmware/co2_sensor.cpp

@@ -10,6 +10,7 @@ namespace config {
   const int8_t max_deviation_during_calibration = 30; // [ppm]
   const int16_t timestep_during_calibration = 10; // [s] WARNING: Measurements can be unreliable for timesteps shorter than 10s.
   const int8_t stable_measurements_before_calibration = 120 / timestep_during_calibration; // [-] Stable measurements during at least 2 minutes.
+  const uint16_t co2_alert_threshold = 2000; // [ppm] Display a flashing led ring, if concentration exceeds this value
 
 #ifdef TEMPERATURE_OFFSET
   // Residual heat from CO2 sensor seems to be high enough to change the temperature reading. How much should it be offset?
@@ -227,11 +228,11 @@ namespace sensor {
      * A short delay is required in order to let background tasks run on the ESP8266.
      * see https://github.com/esp8266/Arduino/issues/3241#issuecomment-301290392
      */
-    if (co2 < 2000) {
+    if (co2 < config::co2_alert_threshold) {
       led_effects::displayCO2color(co2);
       delay(100);
     } else {
-      // >= 2000: entire ring blinks red
+      // Display a flashing led ring, if concentration exceeds a specific value
       led_effects::redAlert();
     }
   }