Add `BUSY_WHILE_HEATING` for hosts that need it

Marlin/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/Marlin_main.cpp

   wait_for_heatup = true;
   millis_t now, next_temp_ms = 0, next_cool_check_ms = 0;
 
-  KEEPALIVE_STATE(NOT_BUSY);
+  #if DISABLED(BUSY_WHILE_HEATING)
+    KEEPALIVE_STATE(NOT_BUSY);
+  #endif
 
   #if ENABLED(PRINTER_EVENT_LEDS)
     const float start_temp = thermalManager.degHotend(target_extruder);
     #endif
   }
 
-  KEEPALIVE_STATE(IN_HANDLER);
+  #if DISABLED(BUSY_WHILE_HEATING)
+    KEEPALIVE_STATE(IN_HANDLER);
+  #endif
 }
 
 #if HAS_TEMP_BED
     wait_for_heatup = true;
     millis_t now, next_temp_ms = 0, next_cool_check_ms = 0;
 
-    KEEPALIVE_STATE(NOT_BUSY);
+    #if DISABLED(BUSY_WHILE_HEATING)
+      KEEPALIVE_STATE(NOT_BUSY);
+    #endif
 
     target_extruder = active_extruder; // for print_heaterstates
 
     } while (wait_for_heatup && TEMP_BED_CONDITIONS);
 
     if (wait_for_heatup) LCD_MESSAGEPGM(MSG_BED_DONE);
-    KEEPALIVE_STATE(IN_HANDLER);
+    #if DISABLED(BUSY_WHILE_HEATING)
+      KEEPALIVE_STATE(IN_HANDLER);
+    #endif
   }
 
 #endif // HAS_TEMP_BED
     if (WITHIN(e, 0, HOTENDS - 1))
       target_extruder = e;
 
-    KEEPALIVE_STATE(NOT_BUSY); // don't send "busy: processing" messages during autotune output
+    #if DISABLED(BUSY_WHILE_HEATING)
+      KEEPALIVE_STATE(NOT_BUSY);
+    #endif
 
     thermalManager.PID_autotune(temp, e, c, u);
 
-    KEEPALIVE_STATE(IN_HANDLER);
+    #if DISABLED(BUSY_WHILE_HEATING)
+      KEEPALIVE_STATE(IN_HANDLER);
+    #endif
   #else
     SERIAL_ERROR_START();
     SERIAL_ERRORLNPGM(MSG_ERR_M303_DISABLED);

Marlin/example_configurations/Anet/A6/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/CL-260/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Cartesio/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Felix/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Felix/DUAL/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/FolgerTech-i3-2020/Configuration.h

 //
 //#define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Hephestos/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Hephestos_2/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 10 // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/Infitary-i3-M508/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/K8200/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/K8400/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/K8400/Dual-head/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/M150/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/RepRapWorld/Megatronics/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/RigidBot/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/SCARA/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/TAZ4/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/TinyBoy2/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/WITBOX/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/adafruit/ST7565/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/FLSUN/auto_calibrate/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 5  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/FLSUN/kossel_mini/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 5  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/generic/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/kossel_mini/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/kossel_pro/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/delta/kossel_xl/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/gCreate_gMax1.5+/Configuration.h

 //
 //#define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/makibox/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/tvrrug/Round2/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher

Marlin/example_configurations/wt150/Configuration.h

 //
 #define HOST_KEEPALIVE_FEATURE        // Disable this if your host doesn't like keepalive messages
 #define DEFAULT_KEEPALIVE_INTERVAL 2  // Number of seconds between "busy" messages. Set with M113.
+#define BUSY_WHILE_HEATING            // Some hosts require "busy" messages even during heating
 
 //
 // M100 Free Memory Watcher