Bluetooth: host: Refactor out BR/EDR from hci_core to br

Refactor out the BR/EDR handling from hci_core.c to its own source file
in br.c.
BR/EDR consists of inquiry and discovery roles in addition to
initialization. SSP is still kept separate.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>