Revert/cleanup mininum extruder height during M600

This change restores the minimum extruder height for filament purge
during M600 from the current 50mm back to 27mm from FW 3.9.

We do this by introducing a new option for unload_filament() to indicate
that the unload is part of an automatic swap, and in such cases avoid
raising more than absolutely necessary (this will _also_ come in handy
to avoid the extra purge in PR #2318 during M600).

A new define MIN_Z_FOR_SWAP is introduced for this purpose.
MIN_Z_FOR_UNLOAD is still used for manual lcd unload and for M702 and
hasn't been changed.

Firmware/Configuration.h

@@ -551,9 +551,10 @@ enum CalibrationStatus
 
 // Try to maintain a minimum distance from the bed even when Z is
 // unknown when doing the following operations
-#define MIN_Z_FOR_LOAD    50
-#define MIN_Z_FOR_UNLOAD  50
-#define MIN_Z_FOR_PREHEAT 10
+#define MIN_Z_FOR_LOAD    50 // lcd filament loading or autoload
+#define MIN_Z_FOR_UNLOAD  50 // lcd filament unloading
+#define MIN_Z_FOR_SWAP    27 // filament change (including M600)
+#define MIN_Z_FOR_PREHEAT 10 // lcd preheat
 
 #include "Configuration_adv.h"
 #include "thermistortables.h"

Firmware/Marlin_main.cpp

@@ -3676,7 +3676,7 @@ static void gcode_M600(bool automatic, float x_position, float y_position, float
 
     // Unload filament
     if (mmu_enabled) extr_unload();	//unload just current filament for multimaterial printers (used also in M702)
-    else unload_filament(); //unload filament for single material (used also in M702)
+    else unload_filament(true); //unload filament for single material (used also in M702)
     //finish moves
     st_synchronize();
 

Firmware/ultralcd.cpp

@@ -4885,7 +4885,7 @@ void lcd_wizard(WizState state)
 				lcd_display_message_fullscreen_P(_i("Now I will preheat nozzle for PLA."));
 				wait_preheat();
 				//unload current filament
-				unload_filament();
+				unload_filament(true);
 				//load filament
 				lcd_wizard_load();
 				setTargetHotend(0, 0); //we are finished, cooldown nozzle
@@ -6200,13 +6200,14 @@ static void change_extr_menu(){
 }
 #endif //SNMM
 
-//unload filament for single material printer (used in M702 gcode)
-void unload_filament()
+// unload filament for single material printer (used in M702 gcode)
+// @param automatic: If true, unload_filament is part of a unload+load sequence (M600)
+void unload_filament(bool automatic)
 {
 	custom_message_type = CustomMsg::FilamentLoading;
 	lcd_setstatuspgm(_T(MSG_UNLOADING_FILAMENT));
 
-    raise_z_above(MIN_Z_FOR_UNLOAD);
+    raise_z_above(automatic? MIN_Z_FOR_SWAP: MIN_Z_FOR_UNLOAD);
 
 	//		extr_unload2();
 

Firmware/ultralcd.h

@@ -195,7 +195,7 @@ extern bool bFilamentAction;
 void mFilamentItem(uint16_t nTemp,uint16_t nTempBed);
 void mFilamentItemForce();
 void lcd_generic_preheat_menu();
-void unload_filament();
+void unload_filament(bool automatic = false);
 
 void stack_error();
 void lcd_printer_connected();