test: use RPL DAG to advertise the dtls server

This adds some code to advertise this server.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
test: print UDP port number when sending IP package

This adds the port number to the contiki dtls client.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
test: print UDP port and IP address when sending package

This adds printing the ip and port number to the contiki dtls server.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
test: change used RPL IP address

The dtls server is using 0xaaa::0200:0000:0000:0001 as its default ip
address.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>