Shut down heaters during BL-Touch probe event

The electro-magnetic interference from the bed and nozzle are affecting
the BL-Touch repeatability for some users.   This problem can be helped
by shutting down the heaters during the actual probe event and then
quickly turning them back on.

Because this code is messing with the heaters, it is written in a
paranoid manner.  It only turns the heaters back on if everything is
EXACTLY as it expects things to be.  The BL-Touch probe must have been
put into a deployed state less than 20 seconds prior, or the stow()
function will NOT turn the heaters on.

This code has been tested and works for both G28 and probing functions.

Marlin/Marlin_main.cpp

       safe_delay(BLTOUCH_DELAY);
     }
 
+    // 
+    // The BL-Touch probes have a HAL effect sensor.  The high currents switching
+    // on and off cause big magnetic fields that can affect the reliability of the
+    // sensor.  So, for BL-Touch probes, we turn off the heaters during the actual probe.
+    // And then we quickly turn them back on after we have sampled the point
+    //
+    void turn_heaters_on_or_off_for_bltouch(const bool deploy) {
+      static int8_t bltouch_recursion_cnt=0;
+      static millis_t last_emi_protection=0;
+      static float temps_at_entry[HOTENDS]; 
+      #if HAS_TEMP_BED
+        static float bed_temp_at_entry;
+      #endif
+
+      if (deploy && bltouch_recursion_cnt>0)         // if already in the correct state, we don't need to do anything
+        return;                                      // with the heaters.
+      if (!deploy && bltouch_recursion_cnt<1)        // if already in the correct state, we don't need to do anything
+        return;                                      // with the heaters.
+
+      if (deploy) {
+        bltouch_recursion_cnt++;
+        last_emi_protection = millis();
+        HOTEND_LOOP() temps_at_entry[e] = thermalManager.degTargetHotend(e);        // save the current target temperatures 
+        HOTEND_LOOP() thermalManager.setTargetHotend(0, e);                         // so we know what to restore them to.
+
+        #if HAS_TEMP_BED
+          bed_temp_at_entry = thermalManager.degTargetBed();
+          thermalManager.setTargetBed(0.0);
+        #endif
+      } 
+      else {
+        bltouch_recursion_cnt--;                                                    // the heaters are only turned back on
+	if (bltouch_recursion_cnt==0 && ((last_emi_protection+20000L)>millis())) {  // if everything is perfect.  It is expected
+          HOTEND_LOOP() thermalManager.setTargetHotend(temps_at_entry[e], e);       // that the bltouch_recursion_cnt is zero and 
+          #if HAS_TEMP_BED                                                          // that the heaters were shut off less than 
+            thermalManager.setTargetBed(bed_temp_at_entry);                         // 20 seconds ago
+          #endif
+        }
+      }
+    }
+
     void set_bltouch_deployed(const bool deploy) {
+      turn_heaters_on_or_off_for_bltouch(deploy);
       if (deploy && TEST_BLTOUCH()) {      // If BL-Touch says it's triggered
         bltouch_command(BLTOUCH_RESET);    // try to reset it.
         bltouch_command(BLTOUCH_DEPLOY);   // Also needs to deploy and stow to
       }
     #endif
 
+    #if ENABLED(BLTOUCH)
+      turn_heaters_on_or_off_for_bltouch(deploy);
+    #endif
+
     if (endstops.z_probe_enabled == deploy) return false;
 
     // Make room for probe