Implement basic HOST_KEEPALIVE_INTERVAL

Marlin/Marlin.h

   extern float zprobe_zoffset;
 #endif
 
+#if ENABLED(HOST_KEEPALIVE_FEATURE)
+  extern uint8_t host_keepalive_interval;
+#endif
+
 #if ENABLED(PREVENT_DANGEROUS_EXTRUDE)
   extern float extrude_min_temp;
 #endif

Marlin/Marlin_main.cpp

 
   static MarlinBusyState busy_state = NOT_BUSY;
   static millis_t next_busy_signal_ms = -1;
+  uint8_t host_keepalive_interval = DEFAULT_KEEPALIVE_INTERVAL;
   #define KEEPALIVE_STATE(n) do{ busy_state = n; }while(0)
 #else
   #define host_keepalive() ;
           break;
       }
     }
-    next_busy_signal_ms = ms + 10000UL; // "busy: ..." message every 10s
+    next_busy_signal_ms = host_keepalive_interval ? ms + 1000UL * host_keepalive_interval : -1;
   }
 
 #endif //HOST_KEEPALIVE_FEATURE