Bluetooth: Merge buf.[ch] functionality into hci_core.[ch]

There's no need to maintain a separate module for this anymore.
Subsequent patches will have further simplifications thanks to this
being now handled in a single c-file.

Change-Id: I4510c9f72b121e1e5fd19eeb8b5d5ddf2f4bfffe
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>