gnrc_uhcpc: add compression context before ABR
gnrc_uhcpc: add compression context before ABR

This way the context is added to the ABR's context list automatically.

OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,ant9000/RIOT,basilfx/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,yogo1212/RIOT,ant9000/RIOT,jasonatran/RIOT,basilfx/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,smlng/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,smlng/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,jasonatran/RIOT,basilfx/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,smlng/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,yogo1212/RIOT,smlng/RIOT,yogo1212/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,smlng/RIOT,miri64/RIOT,authmillenon/RIOT