Fixing FW-256. Problem was that FORWARDING mode is applied to upstream forwarding and downstream forwarding, IPHC was only considering FLOW_LABEL as 3B when Packet was being sent by the node, not when it was forwarded. The workaround had been to look at the NH and see if it is the IANA_IPv6ROUTE which means downstream routing and then use only TF ELIDED, otherwise use the 3B TF.
Merge pull request #88 from openwsn-berkeley/develop_FW-256

FW-256. Flow flow label not relayed during multi-hop upstream transmission.
Merge branch 'develop' into develop_FW-229