Btstack offers two abstraction layers for secret storage, one
called "device db" and another called "tlv". Pairing information
is stored in the "device db", additional secrets (like own keys)
are stored directly in the tlv. Luckily there is a "device db"
implementation using tlv, so we only need to provide one interface.
Additionally, I've removed some btstack files from compilation that
were not referenced.
