Disabled M88 for boards that don't have seperate enable pins.

Marlin/Marlin.pde

@@ -867,6 +867,7 @@ inline void process_commands()
       }
       break;
     case 88: //M88
+      #if ((E_ENABLE_PIN != X_ENABLE_PIN) && (E_ENABLE_PIN != Y_ENABLE_PIN)) // Only enable on boards that have seperate ENABLE_PINS
         if(code_seen('S')) {
           stepper_inactive_time = code_value() * 1000;
         }
@@ -875,6 +876,10 @@ inline void process_commands()
           LCD_MESSAGEPGM("Free Move");
           disable_e();
         }
+      #else
+        SERIAL_ECHO_START;
+        SERIAL_ECHOLN("M88 not supported");
+      #endif
       break;
     case 115: // M115
       SerialprintPGM("FIRMWARE_NAME:Marlin; Sprinter/grbl mashup for gen6 FIRMWARE_URL:http://www.mendel-parts.com PROTOCOL_VERSION:1.0 MACHINE_TYPE:Mendel EXTRUDER_COUNT:1");