ReadMe: fix Readme for Contiki

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: some fixes for recent Contiki OS

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: use same parameter for size in memset

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