dtls: add handshake header after creating the content
dtls: add handshake header after creating the content

In some cases we will not know how big a handshake message will be
before sending it. The handshake header should be added to the message
after the message body was created. This also reduces the code size.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

nmacs/tinydtls,fvdnabee/tinydtls,jvermillard/tinydtls,nmacs/tinydtls,yunus/TinyDTLS-WebID,thecodemaiden/tinydtls-fork,nmacs/tinydtls,spark/tinydtls,thecodemaiden/tinydtls-fork,fvdnabee/tinydtls,nmacs/tinydtls,thecodemaiden/tinydtls-fork,jvermillard/tinydtls,OlegHahm/tinydtls,OlegHahm/tinydtls,yunus/TinyDTLS-WebID,OlegHahm/tinydtls,jvermillard/tinydtls,yunus/TinyDTLS-WebID,spark/tinydtls,fvdnabee/tinydtls,spark/tinydtls