use optional config to allow override flags in referenced project

libs/bluetooth/pxt.json

@@ -16,7 +16,13 @@
         "config": {
             "microbit-dal": {
                 "bluetooth": {
-                    "enabled": 1,
+                    "enabled": 1
+                }
+            }
+        },
+        "optionalConfig": {
+            "microbit-dal": {
+                "bluetooth": {
                     "private_addressing": 0,
                     "whitelist": 1,
                     "advertising_timeout": 0,
@@ -25,19 +31,13 @@
                     "event_service": 1,
                     "device_info_service": 1,
                     "eddystone_url": 1,
-                    "eddystone_uid": 0
-                },
-                "gatt_table_size": "0x700"
-            }
-        },
-        "optionalConfig": {
-            "microbit-dal": {
-                "bluetooth": {
+                    "eddystone_uid": 0,
                     "pairing_mode": 1,
                     "open": 0,
                     "security_level": "SECURITY_MODE_ENCRYPTION_WITH_MITM"
                 }
-            }
+            },
+            "gatt_table_size": "0x700"
         }
     },
     "installedVersion": "vzlhfd"

libs/core/pxt.json

@@ -42,8 +42,7 @@
     "public": true,
     "dependencies": {},
     "yotta": {
-        "configIsJustDefaults": true,
-        "config": {
+        "optionalConfig": {
             "microbit-dal": {
                 "bluetooth": {
                     "enabled": 0