Adjust start g-code to not mix 50/50 by default

This new start g-code has been tested on the A20 by @leshuguets in issue thread #7280.

resources/definitions/geeetech_A10M.def.json

@@ -45,7 +45,7 @@
         "machine_max_jerk_e": { "default_value": 2.5 },
         "machine_gcode_flavor": { "default_value": "RepRap (Marlin/Sprinter)" },
         "machine_start_gcode": {
-            "default_value": "G28 \nG1 Z15 F300\nM107\nG90\nM82\nM104 S215\nM140 S55\nG92 E0\nM109 S215\nM107\nM163 S0 P0.50\nM163 S1 P0.50\nM164 S4\nG0 X10 Y20 F6000\nG1 Z0.8\nG1 F300 X180 E40\nG1 F1200 Z2\nG92 E0\nG28"
+            "default_value": ";GeeeTech A10M start script\nG28 ;home\nG90 ;absolute positioning\nG1 X0 Y0 Z15 E0 F300 ;go to wait position\nM140 S{material_bed_temperature_layer_0} ;set bed temp\nM190 S{material_print_temperature_layer_0} ;set extruder temp and wait\nG1 Z0.8 F200 ;set extruder height\nG1 X220 Y0 E80 F1000 ;purge line\n;end of start script"
         },
         "machine_end_gcode": {
             "default_value": "G91\nG1 E-1\nG0 X0 Y200\nM104 S0\nG90\nG92 E0\nM140 S0\nM84\nM104 S0\nM140 S0\nM84"

resources/definitions/geeetech_A20M.def.json

@@ -45,7 +45,7 @@
         "machine_max_jerk_e": { "default_value": 2.5 },
         "machine_gcode_flavor": { "default_value": "RepRap (Marlin/Sprinter)" },
         "machine_start_gcode": {
-            "default_value": "G28 \nG1 Z15 F300\nM107\nG90\nM82\nM104 S215\nM140 S55\nG92 E0\nM109 S215\nM107\nM163 S0 P0.50\nM163 S1 P0.50\nM164 S4\nG0 X10 Y20 F6000\nG1 Z0.8\nG1 F300 X200 E40\nG1 F1200 Z2\nG92 E0\nG28"
+            "default_value": ";GeeeTech A20M start script\nG28 ;home\nG90 ;absolute positioning\nG1 X0 Y0 Z15 E0 F300 ;go to wait position\nM140 S{material_bed_temperature_layer_0} ;set bed temp\nM190 S{material_print_temperature_layer_0} ;set extruder temp and wait\nG1 Z0.8 F200 ;set extruder height\nG1 X220 Y0 E80 F1000 ;purge line\n;end of start script"
         },
         "machine_end_gcode": {
             "default_value": "G91\nG1 E-1\nG0 X0 Y200\nM104 S0\nG90\nG92 E0\nM140 S0\nM84\nM104 S0\nM140 S0\nM84"