net: ip: 6lo compression rework

This is a rework of the 6lo IPHC. The compression now works inline
on the original buffer instead of allocation a new one.
Additionally DAM_11 (Destination address fully elided) has precedence
over DAM_10 (16 bit compressible) now.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>