Bluetooth: Add bt_buf_get_rx() helper API

The separate ACL & event pools will soon be removed, and it's already
now convenient to have a generic API when the exact type of the
incoming packet is not yet known.

Change-Id: I84cb65d17ea69ebeaeb21532fbf76689e4fb59a0
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>