test: use RPL DAG to advertise the dtls server
test: use RPL DAG to advertise the dtls server

This adds some code to advertise this server.

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

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