Merge Cleanup (PR#2445)

Marlin/Marlin.h

 void FlushSerialRequestResend();
 void ok_to_send();
 
-#ifdef DELTA
-  void calculate_delta(float cartesian[3]);
-  #ifdef ENABLE_AUTO_BED_LEVELING
-    extern int delta_grid_spacing[2];
-    void adjust_delta(float cartesian[3]);
-  #endif
-  extern float delta[3];
-#endif
-#ifdef SCARA
-  void calculate_delta(float cartesian[3]);
-  void calculate_SCARA_forward_Transform(float f_scara[3]);
-#endif
 void reset_bed_level();
 void prepare_move();
 void kill(const char *);
 extern float filament_size[EXTRUDERS]; // cross-sectional area of filament (in millimeters), typically around 1.75 or 2.85, 0 disables the volumetric calculations for the extruder.
 extern float volumetric_multiplier[EXTRUDERS]; // reciprocal of cross-sectional area of filament (in square millimeters), stored this way to reduce computational burden in planner
 extern float current_position[NUM_AXIS];
-extern float home_offset[3];
-
-#ifdef DELTA
-  extern float endstop_adj[3];
-  extern float delta_radius;
-  extern float delta_diagonal_rod;
-  extern float delta_segments_per_second;
-  void recalc_delta_settings(float radius, float diagonal_rod);
-#elif defined(Z_DUAL_ENDSTOPS)
-  extern float z_endstop_adj;
-#endif
+extern float home_offset[3]; // axis[n].home_offset
+extern float min_pos[3]; // axis[n].min_pos
+extern float max_pos[3]; // axis[n].max_pos
+extern bool axis_known_position[3]; // axis[n].is_known
 
-#ifdef SCARA
-  extern float axis_scaling[3];  // Build size scaling
+#if defined(DELTA) || defined(SCARA)
+  void calculate_delta(float cartesian[3]);
+  #ifdef DELTA
+    extern float delta[3];
+    extern float endstop_adj[3]; // axis[n].endstop_adj
+    extern float delta_radius;
+    extern float delta_diagonal_rod;
+    extern float delta_segments_per_second;
+    void recalc_delta_settings(float radius, float diagonal_rod);
+    #ifdef ENABLE_AUTO_BED_LEVELING
+      extern int delta_grid_spacing[2];
+      void adjust_delta(float cartesian[3]);
+    #endif
+  #elif defined(SCARA)
+    extern float axis_scaling[3];  // Build size scaling
+    void calculate_SCARA_forward_Transform(float f_scara[3]);
+  #endif
 #endif
 
-extern float min_pos[3];
-extern float max_pos[3];
-extern bool axis_known_position[3];
+#ifdef Z_DUAL_ENDSTOPS
+  extern float z_endstop_adj;
+#endif
 
 #ifdef ENABLE_AUTO_BED_LEVELING
   extern float zprobe_zoffset;
 
 #ifdef FWRETRACT
   extern bool autoretract_enabled;
-  extern bool retracted[EXTRUDERS];
+  extern bool retracted[EXTRUDERS]; // extruder[n].retracted
   extern float retract_length, retract_length_swap, retract_feedrate, retract_zlift;
   extern float retract_recover_length, retract_recover_length_swap, retract_recover_feedrate;
 #endif

Marlin/temperature.cpp

     static TRState thermal_runaway_state_machine[4] = { TRReset, TRReset, TRReset, TRReset };
     static millis_t thermal_runaway_timer[4]; // = {0,0,0,0};
   #endif
-  #ifdef THERMAL_PROTECTION_BED
+  #if defined(THERMAL_PROTECTION_BED) && TEMP_SENSOR_BED != 0
     static TRState thermal_runaway_bed_state_machine = TRReset;
     static millis_t thermal_runaway_bed_timer;
   #endif
       if (minttemp_raw[0] GE0 current_temperature_raw[0]) min_temp_error(0);
     #endif
 
-    #if HAS_TEMP_1
+    #if HAS_TEMP_1 && EXTRUDERS > 1
       #if HEATER_1_RAW_LO_TEMP > HEATER_1_RAW_HI_TEMP
         #define GE1 <=
       #else
       if (minttemp_raw[1] GE1 current_temperature_raw[1]) min_temp_error(1);
     #endif // TEMP_SENSOR_1
 
-    #if HAS_TEMP_2
+    #if HAS_TEMP_2 && EXTRUDERS > 2
       #if HEATER_2_RAW_LO_TEMP > HEATER_2_RAW_HI_TEMP
         #define GE2 <=
       #else
       if (minttemp_raw[2] GE2 current_temperature_raw[2]) min_temp_error(2);
     #endif // TEMP_SENSOR_2
 
-    #if HAS_TEMP_3
+    #if HAS_TEMP_3 && EXTRUDERS > 3
       #if HEATER_3_RAW_LO_TEMP > HEATER_3_RAW_HI_TEMP
         #define GE3 <=
       #else