Bluetooth: Add functions to manipulate the link key list for SMP

As the LTK (the new type of key being handled now) has more data
associated with it, we need to store this extra data and retrieve
the keys based on that data.

Methods for searching for a key and for adding a new LTK are
introduced here.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>