Changes for BT 2.1

1) Handle incoming 2.1 pairing requests
2) Modify displaying error messages on bond failures.
3) Add delay while accepting incoming pairing for certain 2.1 devices.
When MITM is on, the link key request might come more than once.
Auto accept with a delay.
4) Handle DisplayPasskey callback for pairing a 2.1 keyboard with
a 2.1 device