Overwrite gantry height value instead of default_value

Because fdmprinter now overwrites the value using a formula, the default_value properties are no longer used.

resources/definitions/anet_a6.def.json

@@ -33,7 +33,7 @@
             "default_value": false
         },
         "gantry_height": {
-            "default_value": 55
+            "value": "55"
         },
         "machine_start_gcode": {
             "default_value": "G21 ;metric values\nG90 ;absolute positioning\nM82 ;set extruder to absolute mode\nM107 ;start with the fan off\nG28 X0 Y0 ;move X/Y to min endstops\nG28 Z0 ;move Z to min endstops\nM84 ;steppers off\nM0 S12 ;wait 12 seconds\nM17 ;turn steppers on\nG1 Z10.0 F300 ;move the platform down 10mm\nG92 E0 ;zero the extruded length\nG1 F200 E8 ;extrude 8mm of feed stock\nG92 E0 ;zero the extruded length again\nM0 S5 ;wait 5 seconds\nG1 F9000\nM117 Printing..."

resources/definitions/stereotech_ste320.def.json

@@ -62,7 +62,7 @@
             ]
         },
         "gantry_height": {
-            "default_value": 25
+            "value": "25"
         },
         "machine_use_extruder_offset_to_offset_coords": {
             "default_value": true