This enables using dtls_server with Bluetooth by passing
CONF_FILE=prj_bt.conf, it is also enabled for testing so changes to
dtls-server are actually tested with NETWORKING_WITH_BT.
Change-Id: Ice4f409c8e2d9e97856159482969f6352264864b
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>