Add 512 padding bytes to the end of the g-code

Due to a bug in the firmware of the Ultimaker 2, the print was already ending before the last few commands were executed completely. It would then finish the last few commands in a different routine but the extrusion modifier wasn't applied there, resulting in a blob on your print.
This is a workaround that puts always one file system page (512 bytes) of padding at the end of the g-code file. If these comments are not executed that is fine. If they are then it's also fine. Until the bug is fixed in firmware, this will at least prevent the issue from occurring for users.

resources/definitions/ultimaker2.def.json

@@ -20,12 +20,10 @@
     "overrides": {
         "machine_name": { "default_value": "Ultimaker 2" },
         "machine_start_gcode" : {
-            "default_value": "",
             "value": "\"\"  if machine_gcode_flavor == \"UltiGCode\" else \"G21 ;metric values\\nG90 ;absolute positioning\\nM82 ;set extruder to absolute mode\\nM107 ;start with the fan off\\nG28 Z0 ;move Z to bottom endstops\\nG28 X0 Y0 ;move X/Y to endstops\\nG1 X15 Y0 F4000 ;move X/Y to front of printer\\nG1 Z15.0 F9000 ;move the platform to 15mm\\nG92 E0 ;zero the extruded length\\nG1 F200 E10 ;extrude 10 mm of feed stock\\nG92 E0 ;zero the extruded length again\\nG1 F9000\\n;Put printing message on LCD screen\\nM117 Printing...\""
         },
         "machine_end_gcode" : {
-            "default_value": "",
-            "value": "\"\"  if machine_gcode_flavor == \"UltiGCode\" else \"M104 S0 ;extruder heater off\\nM140 S0 ;heated bed heater off (if you have it)\\nG91 ;relative positioning\\nG1 E-1 F300  ;retract the filament a bit before lifting the nozzle, to release some of the pressure\\nG1 Z+0.5 E-5 X-20 Y-20 F9000 ;move Z up a bit and retract filament even more\\nG28 X0 Y0 ;move X/Y to min endstops, so the head is out of the way\\nM84 ;steppers off\\nG90 ;absolute positioning\""
+            "value": "\";Version _2.6 of the firmware can abort the print too early if the file ends\\n;too soon. However if the file hasn't ended yet because there are comments at\\n;the end of the file, it won't abort yet. Therefore we have to put at least 512\\n;bytes at the end of the g-code so that the file is not yet finished by the\\n;time that the motion planner gets flushed. With firmware version _3.3 this\\n;should be fixed, so this comment wouldn't be necessary any more. Now we have\\n;to pad this text to make precisely 512 bytes.\"  if machine_gcode_flavor == \"UltiGCode\" else \"M104 S0 ;extruder heater off\\nM140 S0 ;heated bed heater off (if you have it)\\nG91 ;relative positioning\\nG1 E-1 F300  ;retract the filament a bit before lifting the nozzle, to release some of the pressure\\nG1 Z+0.5 E-5 X-20 Y-20 F9000 ;move Z up a bit and retract filament even more\\nG28 X0 Y0 ;move X/Y to min endstops, so the head is out of the way\\nM84 ;steppers off\\nG90 ;absolute positioning\\n;Version _2.6 of the firmware can abort the print too early if the file ends\\n;too soon. However if the file hasn't ended yet because there are comments at\\n;the end of the file, it won't abort yet. Therefore we have to put at least 512\\n;bytes at the end of the g-code so that the file is not yet finished by the\\n;time that the motion planner gets flushed. With firmware version _3.3 this\\n;should be fixed, so this comment wouldn't be necessary any more. Now we have\\n;to pad this text to make precisely 512 bytes.\""
         },
         "machine_width": {
             "default_value": 223