Allow for using a fixed frame header length.
Allow for using a fixed frame header length.

When SICSLOWPAN_FRAMER_HDRLEN is defined its value is used
as a frame header length. This allows for using sicslowpan
without calling a framer. This is usefull if framer is not
used on a given platform or when header length is always
the same.

In addition this commit also fixes an inline define.

MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki