dtls: add psk decrypt support

This adds support for decryption of DTLS packages when a PSK cipher was
used.

The code for generating a pre master secret was moved from packet-ssl.c
into packet-ssl-utils.c and packet-dtls.c now uses the same code to
generate a pre master secret.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
tmp: add basic support for TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and raw public keys

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