rpl: cleanup, changing structdefs into typedefs
Merge pull request #2234 from cgundogan/rpl_struct_typedef_cleanup

rpl: cleanup, changing structdefs into typedefs
rpl: wrong offset for dao-ack receive buffer

Every other buffer is using the offset `IPV6_HDR_LEN + x`
and the `rpl_buffer` variable instead of the `buffer` variable.
Even the send buffer for dao-acks.
Adding another `LL_HDR_LEN` to the dao-ack's recv buffer offset
seems obsolete and faulty.