Fix for RPL Hop-by-Hop options

Currently, RPL Hop-by-Hop option is only inserted for forwarded packets, this option must be also present for first hop packets. Also the sender rank field inside the HBH option is not in network format as required by RFC.

This PR fixes the sender rank in the option and properly insert the option on all the outgoing packets. Also, HBH option must be inserted after UDP CRC calculation or the CRC is calculated using the wrong part of the IPv6 header.