Bluetooth: Add flag for LE GAP Peripheral role

This patch adds a flag to be used for LE GAP Peripheral role. In this
role undirected advertising will be enabled and operations not allowed
in Peripheral role (such as scanning and initiating connections) will be
disallowed.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>