dtls: fix typo in CERTIFICATE

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

The certificate type is now officially named Raw Public Key

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: replace it with the keys from IANA

The IANA assigned some official values for this draft, see
https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml

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