put the acl buffer size in config

Author: paul-szczepanek-arm

features/FEATURE_BLE/targets/TARGET_CORDIO/mbed_lib.json

@@ -51,6 +51,10 @@
             "help": "Desired ATT_MTU, this needs to be between 23 and 517 (inclusive).",
             "value": 23
         },
+        "rx-acl-buffer-size": {
+            "help": "Size of the buffer holding the ACL packet. This will limit the effective ATT_MTU (to its value minus 4 bytes for the header). The size of the buffer must be small enough to be allocated from the existing cordio pool. If this value is increased you may need to adjust the memory pool.",
+            "value": 100
+        },
         "max-prepared-writes": {
             "help": "Number of queued prepare writes supported by server.",
             "value": 4

features/FEATURE_BLE/targets/TARGET_CORDIO/source/CordioBLE.cpp

@@ -510,7 +510,7 @@ void BLE::stack_setup()
 
     stack_handler_id = WsfOsSetNextHandler(&BLE::stack_handler);
 
-    HciSetMaxRxAclLen(100);
+    HciSetMaxRxAclLen(MBED_CONF_CORDIO_RX_ACL_BUFFER_SIZE);
 
     DmRegister(BLE::device_manager_cb);
 #if BLE_FEATURE_CONNECTABLE