Update diymore_f407vgt.json (#545)

Flash size in F407VG is 1024K

Author: tute-avalos

boards/diymore_f407vgt.json

@@ -25,7 +25,7 @@
   "name": "F407VG",
   "upload": {
     "maximum_ram_size": 131072,
-    "maximum_size": 524288,
+    "maximum_size": 1048576,
     "protocol": "stlink",
     "protocols": [
       "jlink",