test: add ECC support to contiki example

This adds ECC keys to the dtls-client.c and dtls-server.c used in the
contiki example. This makes it possible to perform a ECDH_ECDSA cipher.
The values are copied from the Linux example.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
test: make contiki dtls example server send package back

When the contiki dtls example server receives a package, it now sends it
back to the client. This is now done like the Linux server does it.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: to not send alert when CCS was wrong

When the ChangeCipherSpec is received, but was not expected do not send
an alert, but ignore it. This could happen when a previous handshake
package got lost. The CCS should be stored for reordering in a next
patch.

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