Add "SINGLENOZZLE" extruder option

Marlin/example_configurations/adafruit/ST7565/Configuration.h

@@ -120,6 +120,9 @@
 // :[1,2,3,4]
 #define EXTRUDERS 1
 
+// For Cyclops or any "multi-extruder" that shares a single nozzle.
+//#define SINGLENOZZLE
+
 // Offset of the extruders (uncomment if using more than one and relying on firmware to position when changing).
 // The offset has to be X=0, Y=0 for the extruder 0 hotend (default extruder).
 // For the other hotends it is their distance from the extruder 0 hotend.