ll: Rename pdu_type to type

This patch removes redundant prefix in the advertising type field.
ll: Add scannable advertising

This patch adds support for scannable advertising, sending SCAN_RSP if
SCAN_REQ PDU is received. SCAN_REQ PDUs can be sent by the remote if the
advertiser is sending ADV_IND or ADV_SCAN_IND.
example: Add setting scan response data

This patch changes the ll-broadcaster example to send scannable
advertising PDU, and reply device appearance in the scan response
PDU.