Removed the DISABLE from HOST_KEEPALIVE_FEATURE option

Marlin/Conditionals.h

   #endif
 
   /**
-   * Avoid double-negatives for enabling features
+   * Host keep alive
    */
-  #if DISABLED(DISABLE_HOST_KEEPALIVE)
-    #define HOST_KEEPALIVE_FEATURE
+  #ifndef HOST_KEEPALIVE_INTERVAL
+    #define HOST_KEEPALIVE_INTERVAL 2
   #endif
 
   /**

Marlin/Configuration.h

 //
 // Host Keepalive
 //
-// By default Marlin will send a busy status message to the host
+// When enabled Marlin will send a busy status message to the host
 // every couple of seconds when it can't accept commands.
 //
-//#define DISABLE_HOST_KEEPALIVE // Enable this option if your host doesn't like keepalive messages.
-#if DISABLED(DISABLE_HOST_KEEPALIVE)
-  #define DEFAULT_KEEPALIVE_INTERVAL 2 // Number of seconds between "busy" messages. Set with M113.
-#endif
+#define HOST_KEEPALIVE_FEATURE    // Disable this if your host doesn't like keepalive messages
+#define HOST_KEEPALIVE_INTERVAL 2 // Number of seconds between "busy" messages. Set with M113.
 
 //
 // M100 Free Memory Watcher

Marlin/Marlin_main.cpp

 
   static MarlinBusyState busy_state = NOT_BUSY;
   static millis_t next_busy_signal_ms = 0;
-  uint8_t host_keepalive_interval = DEFAULT_KEEPALIVE_INTERVAL;
+  uint8_t host_keepalive_interval = HOST_KEEPALIVE_INTERVAL;
   #define KEEPALIVE_STATE(n) do{ busy_state = n; }while(0)
 #else
   #define host_keepalive() ;