PX4 FLOW: Store maximum flash size

Images/px4flow-v2.prototype

 {
     "board_id": 24,
     "magic": "FLOWv1",
-    "description": "Firmware for the PX4FLowV1 board",
+    "description": "Firmware for the PX4FlowV2 board",
     "image": "",
     "build_time": 0,
-    "summary": "PX4FLOWv1",
+    "summary": "PX4FLOWv2",
     "version": "0.1",
     "image_size": 0,
+    "image_maxsize": 2080768,
     "git_identity": "",
     "board_revision": 0
 }