Send a Busy signal to hosts during long processes, wait for input, etc

If Marlin is blocking the serial input or command queue for any length
of time (for example more than 2 seconds), it needs to send a message
to serial out to inform the host that it is busy. Marlin should only
send these messages out when busy, and preferably not when trying to
print formatted output.

Marlin/Conditionals.h

@@ -314,6 +314,13 @@
     #define Z_SAFE_HOMING
   #endif
 
+  /**
+   * Avoid double-negatives for enabling features
+   */
+  #if DISABLED(DISABLE_HOST_KEEPALIVE)
+    #define HOST_KEEPALIVE_FEATURE
+  #endif
+
   /**
    * MAX_STEP_FREQUENCY differs for TOSHIBA
    */