Added fixups for msgb len field whenever the tail is modified
Also, added some clarifying comments
Change-Id: Ia725edbeafe26bd2ea9b5a1810d0b26bc79d84db
the rx_tm_sdu and rx_tl_sdu did not belong in tetra_upper_mac and are removed. Instead, we use rx_tm_sdu in tetra_llc.c, who in turn invokes rx_tl_sdu in tetra_mle.c. The llc can now also make use of the advanced link defragmentation code that was already there but unused.
Change-Id: I294c684e97c55876f1a207a7152a83dad4ebaa26