Fix up, improve endstop pin auto-assignment

Followup for #16723

Marlin/src/pins/lpc1768/pins_BTT_SKR_V1_3.h

  * Limit Switches
  */
 #if X_STALL_SENSITIVITY
+  #define X_STOP_PIN         X_DIAG_PIN
   #if X_HOME_DIR < 0
-    #define X_MIN_PIN        X_DIAG_PIN
     #define X_MAX_PIN        P1_28   // X+
   #else
-    #define X_MAX_PIN        X_DIAG_PIN
     #define X_MIN_PIN        P1_28   // X+
   #endif
 #else
 #endif
 
 #if Y_STALL_SENSITIVITY
+  #define Y_STOP_PIN         Y_DIAG_PIN
   #if Y_HOME_DIR < 0
-    #define Y_MIN_PIN        Y_DIAG_PIN
     #define Y_MAX_PIN        P1_26   // Y+
   #else
-    #define Y_MAX_PIN        Y_DIAG_PIN
     #define Y_MIN_PIN        P1_26   // Y+
   #endif
 #else
 #endif
 
 #if Z_STALL_SENSITIVITY
+  #define Z_STOP_PIN         Z_DIAG_PIN
   #if Z_HOME_DIR < 0
-    #define Z_MIN_PIN        Z_DIAG_PIN
     #define Z_MAX_PIN        P1_24   // Z+
   #else
-    #define Z_MAX_PIN        Z_DIAG_PIN
     #define Z_MIN_PIN        P1_24   // Z+
   #endif
 #else
   #define E1_SERIAL_TX_PIN P1_04
   #define E1_SERIAL_RX_PIN P1_01
 
-  #define Z2_SERIAL_TX_PIN P1_04
-  #define Z2_SERIAL_RX_PIN P1_01
-
   // Reduce baud rate to improve software serial reliability
   #define TMC_BAUD_RATE 19200
 #endif

Marlin/src/pins/lpc1768/pins_BTT_SKR_V1_4.h

 // Limit Switches
 //
 #if X_STALL_SENSITIVITY
+  #define X_STOP_PIN       X_DIAG_PIN
   #if X_HOME_DIR < 0
-    #define X_MIN_PIN      X_DIAG_PIN
     #define X_MAX_PIN      P1_26   // E0DET
   #else
-    #define X_MAX_PIN      X_DIAG_PIN
     #define X_MIN_PIN      P1_26   // E0DET
   #endif
 #else
 #endif
 
 #if Y_STALL_SENSITIVITY
+  #define Y_STOP_PIN       Y_DIAG_PIN
   #if Y_HOME_DIR < 0
-    #define Y_MIN_PIN      Y_DIAG_PIN
     #define Y_MAX_PIN      P1_25   // E1DET
   #else
-    #define Y_MAX_PIN      Y_DIAG_PIN
     #define Y_MIN_PIN      P1_25   // E1DET
   #endif
 #else
 #endif
 
 #if Z_STALL_SENSITIVITY
+  #define Z_STOP_PIN       Z_DIAG_PIN
   #if Z_HOME_DIR < 0
-    #define Z_MIN_PIN      Z_DIAG_PIN
     #define Z_MAX_PIN      P1_24   // PWRDET
   #else
-    #define Z_MAX_PIN      Z_DIAG_PIN
     #define Z_MIN_PIN      P1_24   // PWRDET
   #endif
 #else

Marlin/src/pins/pins.h

 #ifdef X_STOP_PIN
   #if X_HOME_DIR < 0
     #define X_MIN_PIN X_STOP_PIN
-    #define X_MAX_PIN -1
+    #ifndef X_MAX_PIN
+      #define X_MAX_PIN -1
+    #endif
   #else
-    #define X_MIN_PIN -1
     #define X_MAX_PIN X_STOP_PIN
+    #ifndef X_MIN_PIN
+      #define X_MIN_PIN -1
+    #endif
   #endif
 #elif X_HOME_DIR < 0
   #define X_STOP_PIN X_MIN_PIN
 #ifdef Y_STOP_PIN
   #if Y_HOME_DIR < 0
     #define Y_MIN_PIN Y_STOP_PIN
-    #define Y_MAX_PIN -1
+    #ifndef Y_MAX_PIN
+      #define Y_MAX_PIN -1
+    #endif
   #else
-    #define Y_MIN_PIN -1
     #define Y_MAX_PIN Y_STOP_PIN
+    #ifndef Y_MIN_PIN
+      #define Y_MIN_PIN -1
+    #endif
   #endif
 #elif Y_HOME_DIR < 0
   #define Y_STOP_PIN Y_MIN_PIN
 #ifdef Z_STOP_PIN
   #if Z_HOME_DIR < 0
     #define Z_MIN_PIN Z_STOP_PIN
-    #define Z_MAX_PIN -1
+    #ifndef Z_MAX_PIN
+      #define Z_MAX_PIN -1
+    #endif
   #else
-    #define Z_MIN_PIN -1
     #define Z_MAX_PIN Z_STOP_PIN
+    #ifndef Z_MIN_PIN
+      #define Z_MIN_PIN -1
+    #endif
   #endif
 #elif Z_HOME_DIR < 0
   #define Z_STOP_PIN Z_MIN_PIN
  * overridden in Configuration.h or Configuration_adv.h.
  */
 
+#define __PEXI(p,q) PIN_EXISTS(E##p##_##q)
+#define _PEXI(p,q) __PEXI(p,q)
 #define __EPIN(p,q) E##p##_##q##_PIN
 #define _EPIN(p,q) __EPIN(p,q)
-#define DIAG_REMAPPED(P,Q) (PIN_EXISTS(Q) && (P##_DIAG_PIN == Q##_PIN))
+#define DIAG_REMAPPED(p,q) (PIN_EXISTS(q) && _EPIN(p##_E_INDEX, DIAG) == q##_PIN)
 
 // The X2 axis, if any, should be the next open extruder port
 #define X2_E_INDEX E_STEPPERS
   //
   // Auto-assign pins for stallGuard sensorless homing
   //
-  #if X2_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _EPIN(X2_E_INDEX, DIAG) > 0
+  #if X2_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _PEXI(X2_E_INDEX, DIAG)
+    #define X2_DIAG_PIN _EPIN(X2_E_INDEX, DIAG)
     #if   DIAG_REMAPPED(X2, X_MIN)      // If already remapped in the pins file...
       #define X2_USE_ENDSTOP _XMIN_
     #elif DIAG_REMAPPED(X2, Y_MIN)
       #define _X2_USE_ENDSTOP(P) _E##P##_DIAG_
       #define X2_USE_ENDSTOP _X2_USE_ENDSTOP(X2_E_INDEX)
     #endif
+    #undef X2_DIAG_PIN
   #endif
 
   #define Y2_E_INDEX INCREMENT(X2_E_INDEX)
       #define Y2_SERIAL_RX_PIN _EPIN(Y2_E_INDEX, SERIAL_RX)
     #endif
   #endif
-  #if Y2_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _EPIN(Y2_E_INDEX, DIAG) > 0
+  #if Y2_STALL_SENSITIVITY && ENABLED(Y_DUAL_ENDSTOPS) && _PEXI(Y2_E_INDEX, DIAG)
+    #define Y2_DIAG_PIN _EPIN(Y2_E_INDEX, DIAG)
     #if   DIAG_REMAPPED(Y2, X_MIN)
       #define Y2_USE_ENDSTOP _XMIN_
     #elif DIAG_REMAPPED(Y2, Y_MIN)
       #define _Y2_USE_ENDSTOP(P) _E##P##_DIAG_
       #define Y2_USE_ENDSTOP _Y2_USE_ENDSTOP(Y2_E_INDEX)
     #endif
+    #undef Y2_DIAG_PIN
   #endif
   #define Z2_E_INDEX INCREMENT(Y2_E_INDEX)
 #else
       #define Z2_SERIAL_RX_PIN _EPIN(Z2_E_INDEX, SERIAL_RX)
     #endif
   #endif
-  #if Z2_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _EPIN(Z2_E_INDEX, DIAG) > 0
+  #if Z2_STALL_SENSITIVITY && ENABLED(Z_MULTI_ENDSTOPS) && NUM_Z_STEPPER_DRIVERS >= 2 && _PEXI(Z2_E_INDEX, DIAG)
+    #define Z2_DIAG_PIN _EPIN(Z2_E_INDEX, DIAG)
     #if   DIAG_REMAPPED(Z2, X_MIN)
       #define Z2_USE_ENDSTOP _XMIN_
     #elif DIAG_REMAPPED(Z2, Y_MIN)
       #define _Z2_USE_ENDSTOP(P) _E##P##_DIAG_
       #define Z2_USE_ENDSTOP _Z2_USE_ENDSTOP(Z2_E_INDEX)
     #endif
+    #undef Z2_DIAG_PIN
   #endif
   #define Z3_E_INDEX INCREMENT(Z2_E_INDEX)
 #else
       #define Z3_SERIAL_RX_PIN _EPIN(Z3_E_INDEX, SERIAL_RX)
     #endif
   #endif
-  #if Z3_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _EPIN(Z3_E_INDEX, DIAG) > 0
+  #if Z3_STALL_SENSITIVITY && ENABLED(Z_MULTI_ENDSTOPS) && NUM_Z_STEPPER_DRIVERS >= 3 && _PEXI(Z3_E_INDEX, DIAG)
+    #define Z3_DIAG_PIN _EPIN(Z3_E_INDEX, DIAG)
     #if   DIAG_REMAPPED(Z3, X_MIN)
       #define Z3_USE_ENDSTOP _XMIN_
     #elif DIAG_REMAPPED(Z3, Y_MIN)
       #define Z3_USE_ENDSTOP _ZMAX_
     #else
       #define _Z3_USE_ENDSTOP(P) _E##P##_DIAG_
-      #define Z3_USE_ENDSTOP _Y2_USE_ENDSTOP(Y2_E_INDEX)
+      #define Z3_USE_ENDSTOP _Z3_USE_ENDSTOP(Z3_E_INDEX)
     #endif
+    #undef Z3_DIAG_PIN
   #endif
   #define Z4_E_INDEX INCREMENT(Z3_E_INDEX)
 #endif
       #define Z4_SERIAL_RX_PIN _EPIN(Z4_E_INDEX, SERIAL_RX)
     #endif
   #endif
-  #if Z4_STALL_SENSITIVITY && ENABLED(X_DUAL_ENDSTOPS) && _EPIN(Z4_E_INDEX, DIAG) > 0
+  #if Z4_STALL_SENSITIVITY && ENABLED(Z_MULTI_ENDSTOPS) && NUM_Z_STEPPER_DRIVERS >= 4 && _PEXI(Z4_E_INDEX, DIAG)
+    #define Z4_DIAG_PIN _EPIN(Z4_E_INDEX, DIAG)
     #if   DIAG_REMAPPED(Z4, X_MIN)
       #define Z4_USE_ENDSTOP _XMIN_
     #elif DIAG_REMAPPED(Z4, Y_MIN)
       #define _Z4_USE_ENDSTOP(P) _E##P##_DIAG_
       #define Z4_USE_ENDSTOP _Z4_USE_ENDSTOP(Z4_E_INDEX)
     #endif
+    #undef Z4_DIAG_PIN
   #endif
 #endif