gcoap: use generic netreg initializers
gcoap: use generic netreg initializers

PR #5526 introduced mbox support for netreg so using these functions
is now required.

rfuentess/RIOT,jbeyerstedt/RIOT-OTA-update,Hyungsin/RIOT-OS,A-Paul/RIOT,adrianghc/RIOT,backenklee/RIOT,LudwigKnuepfer/RIOT,neiljay/RIOT,kbumsik/RIOT,Hyungsin/RIOT-OS,x3ro/RIOT,toonst/RIOT,neiljay/RIOT,miri64/RIOT,katezilla/RIOT,x3ro/RIOT,dkm/RIOT,kerneltask/RIOT,Josar/RIOT,Ell-i/RIOT,jbeyerstedt/RIOT-OTA-update,LudwigOrtmann/RIOT,gebart/RIOT,kb2ma/RIOT,BytesGalore/RIOT,gebart/RIOT,kerneltask/RIOT,neiljay/RIOT,cladmi/RIOT,miri64/RIOT,rfuentess/RIOT,ant9000/RIOT,roberthartung/RIOT,ks156/RIOT,hamilton-mote/RIOT-OS,BytesGalore/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,basilfx/RIOT,neumodisch/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,A-Paul/RIOT,biboc/RIOT,toonst/RIOT,kYc0o/RIOT,aeneby/RIOT,katezilla/RIOT,jfischer-phytec-iot/RIOT,A-Paul/RIOT,yogo1212/RIOT,syin2/RIOT,thomaseichinger/RIOT,hamilton-mote/RIOT-OS,OTAkeys/RIOT,syin2/RIOT,Ell-i/RIOT,dailab/RIOT,alignan/RIOT,yogo1212/RIOT,immesys/RiSyn,rfuentess/RIOT,cladmi/RIOT,tfar/RIOT,jfischer-phytec-iot/RIOT,cladmi/RIOT,kb2ma/RIOT,BytesGalore/RIOT,mtausig/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,plushvoxel/RIOT,syin2/RIOT,kYc0o/RIOT,yogo1212/RIOT,miri64/RIOT,kaspar030/RIOT,tfar/RIOT,avmelnikoff/RIOT,backenklee/RIOT,RIOT-OS/RIOT,TobiasFredersdorf/RIOT,aeneby/RIOT,BytesGalore/RIOT,mfrey/RIOT,lazytech-org/RIOT,TobiasFredersdorf/RIOT,plushvoxel/RIOT,basilfx/RIOT,jasonatran/RIOT,jfischer-phytec-iot/RIOT,roberthartung/RIOT,thomaseichinger/RIOT,aeneby/RIOT,jfischer-phytec-iot/RIOT,alignan/RIOT,LudwigKnuepfer/RIOT,mfrey/RIOT,ant9000/RIOT,mtausig/RIOT,biboc/RIOT,lazytech-org/RIOT,RIOT-OS/RIOT,hamilton-mote/RIOT-OS,kYc0o/RIOT,backenklee/RIOT,x3ro/RIOT,toonst/RIOT,kerneltask/RIOT,katezilla/RIOT,dailab/RIOT,gautric/RIOT,authmillenon/RIOT,beurdouche/RIOT,kYc0o/RIOT,Hyungsin/RIOT-OS,kaspar030/RIOT,josephnoir/RIOT,neiljay/RIOT,roberthartung/RIOT,LudwigKnuepfer/RIOT,Josar/RIOT,ant9000/RIOT,gautric/RIOT,lazytech-org/RIOT,adjih/RIOT,ks156/RIOT,ks156/RIOT,hamilton-mote/RIOT-OS,kaspar030/RIOT,immesys/RiSyn,alignan/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,gebart/RIOT,smlng/RIOT,beurdouche/RIOT,kbumsik/RIOT,kaleb-himes/RIOT,jfischer-phytec-iot/RIOT,jasonatran/RIOT,thomaseichinger/RIOT,kb2ma/RIOT,Josar/RIOT,dailab/RIOT,immesys/RiSyn,rfuentess/RIOT,josephnoir/RIOT,adjih/RIOT,dailab/RIOT,cladmi/RIOT,tfar/RIOT,mfrey/RIOT,gautric/RIOT,adrianghc/RIOT,ant9000/RIOT,backenklee/RIOT,A-Paul/RIOT,rfuentess/RIOT,Ell-i/RIOT,x3ro/RIOT,beurdouche/RIOT,aeneby/RIOT,plushvoxel/RIOT,katezilla/RIOT,josephnoir/RIOT,basilfx/RIOT,OlegHahm/RIOT,thomaseichinger/RIOT,Hyungsin/RIOT-OS,lazytech-org/RIOT,yogo1212/RIOT,jbeyerstedt/RIOT-OTA-update,mfrey/RIOT,kerneltask/RIOT,adrianghc/RIOT,Hyungsin/RIOT-OS,LudwigKnuepfer/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,immesys/RiSyn,jasonatran/RIOT,LudwigOrtmann/RIOT,alignan/RIOT,mtausig/RIOT,gautric/RIOT,toonst/RIOT,LudwigOrtmann/RIOT,neumodisch/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,biboc/RIOT,OTAkeys/RIOT,ks156/RIOT,roberthartung/RIOT,backenklee/RIOT,Josar/RIOT,basilfx/RIOT,hamilton-mote/RIOT-OS,neiljay/RIOT,katezilla/RIOT,biboc/RIOT,gautric/RIOT,tfar/RIOT,TobiasFredersdorf/RIOT,josephnoir/RIOT,kaspar030/RIOT,neumodisch/RIOT,neumodisch/RIOT,authmillenon/RIOT,kbumsik/RIOT,OTAkeys/RIOT,kb2ma/RIOT,LudwigOrtmann/RIOT,Ell-i/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,immesys/RiSyn,jbeyerstedt/RIOT-OTA-update,aeneby/RIOT,TobiasFredersdorf/RIOT,basilfx/RIOT,miri64/RIOT,mfrey/RIOT,RIOT-OS/RIOT,jbeyerstedt/RIOT-OTA-update,miri64/RIOT,OlegHahm/RIOT,avmelnikoff/RIOT,dkm/RIOT,kaleb-himes/RIOT,yogo1212/RIOT,cladmi/RIOT,ant9000/RIOT,kb2ma/RIOT,adrianghc/RIOT,gebart/RIOT,OlegHahm/RIOT,syin2/RIOT,LudwigOrtmann/RIOT,OTAkeys/RIOT,adrianghc/RIOT,jasonatran/RIOT,OTAkeys/RIOT,ks156/RIOT,mtausig/RIOT,BytesGalore/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,Josar/RIOT,tfar/RIOT,avmelnikoff/RIOT,LudwigOrtmann/RIOT,thomaseichinger/RIOT,beurdouche/RIOT,yogo1212/RIOT,kYc0o/RIOT,OlegHahm/RIOT,toonst/RIOT,LudwigKnuepfer/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,kerneltask/RIOT,dailab/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,A-Paul/RIOT,neumodisch/RIOT,plushvoxel/RIOT,roberthartung/RIOT,biboc/RIOT,beurdouche/RIOT,gebart/RIOT,Ell-i/RIOT,authmillenon/RIOT,kbumsik/RIOT,neumodisch/RIOT,alignan/RIOT,dkm/RIOT,kaleb-himes/RIOT,smlng/RIOT,adjih/RIOT,mtausig/RIOT,dkm/RIOT,kaleb-himes/RIOT,smlng/RIOT,immesys/RiSyn,smlng/RIOT,authmillenon/RIOT,plushvoxel/RIOT,kbumsik/RIOT,adjih/RIOT,dkm/RIOT,adjih/RIOT,smlng/RIOT,syin2/RIOT,x3ro/RIOT,kaleb-himes/RIOT,TobiasFredersdorf/RIOT