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

Marlin/Marlin.pde

@@ -867,14 +867,19 @@ inline void process_commands()
       }
       break;
     case 88: //M88
-      if(code_seen('S')) {
-        stepper_inactive_time = code_value() * 1000;
-      }
-      else {
-        st_synchronize()
-        LCD_MESSAGEPGM("Free Move");
-        disable_e();
-      }
+      #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;
+        }
+        else {
+          st_synchronize()
+          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");