gnrc_netif: generalize IID generation for short addresses
gnrc_netif: generalize IID generation for short addresses

Moves setting of addresses of length lesser than 3 to a function that
initializes the first 5 byte of the IID with `00:00:00:ff:fe` and then
fills up the rest with zeroes and the given short address.

neiljay/RIOT,mfrey/RIOT,smlng/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,lazytech-org/RIOT,aeneby/RIOT,roberthartung/RIOT,rfuentess/RIOT,mfrey/RIOT,authmillenon/RIOT,smlng/RIOT,aeneby/RIOT,x3ro/RIOT,yogo1212/RIOT,OlegHahm/RIOT,Josar/RIOT,neiljay/RIOT,basilfx/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,rfuentess/RIOT,biboc/RIOT,LudwigOrtmann/RIOT,miri64/RIOT,cladmi/RIOT,kbumsik/RIOT,gebart/RIOT,Josar/RIOT,toonst/RIOT,biboc/RIOT,Josar/RIOT,biboc/RIOT,aeneby/RIOT,kYc0o/RIOT,authmillenon/RIOT,biboc/RIOT,RIOT-OS/RIOT,kbumsik/RIOT,kaspar030/RIOT,jasonatran/RIOT,mtausig/RIOT,ant9000/RIOT,smlng/RIOT,toonst/RIOT,smlng/RIOT,avmelnikoff/RIOT,toonst/RIOT,A-Paul/RIOT,lazytech-org/RIOT,basilfx/RIOT,Josar/RIOT,mtausig/RIOT,kbumsik/RIOT,miri64/RIOT,basilfx/RIOT,yogo1212/RIOT,josephnoir/RIOT,x3ro/RIOT,OTAkeys/RIOT,BytesGalore/RIOT,Josar/RIOT,jasonatran/RIOT,jasonatran/RIOT,cladmi/RIOT,cladmi/RIOT,neiljay/RIOT,miri64/RIOT,yogo1212/RIOT,LudwigKnuepfer/RIOT,A-Paul/RIOT,roberthartung/RIOT,miri64/RIOT,A-Paul/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,gebart/RIOT,ant9000/RIOT,mtausig/RIOT,kaspar030/RIOT,josephnoir/RIOT,basilfx/RIOT,OlegHahm/RIOT,LudwigKnuepfer/RIOT,LudwigOrtmann/RIOT,BytesGalore/RIOT,kYc0o/RIOT,mfrey/RIOT,A-Paul/RIOT,avmelnikoff/RIOT,gebart/RIOT,LudwigOrtmann/RIOT,neiljay/RIOT,roberthartung/RIOT,LudwigOrtmann/RIOT,lazytech-org/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,basilfx/RIOT,aeneby/RIOT,authmillenon/RIOT,lazytech-org/RIOT,mtausig/RIOT,LudwigKnuepfer/RIOT,A-Paul/RIOT,toonst/RIOT,aeneby/RIOT,kbumsik/RIOT,rfuentess/RIOT,x3ro/RIOT,roberthartung/RIOT,x3ro/RIOT,josephnoir/RIOT,LudwigKnuepfer/RIOT,rfuentess/RIOT,jasonatran/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,josephnoir/RIOT,mtausig/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,rfuentess/RIOT,yogo1212/RIOT,cladmi/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,OTAkeys/RIOT,jasonatran/RIOT,LudwigKnuepfer/RIOT,OlegHahm/RIOT,yogo1212/RIOT,LudwigOrtmann/RIOT,BytesGalore/RIOT,kbumsik/RIOT,gebart/RIOT,cladmi/RIOT,toonst/RIOT,kYc0o/RIOT,gebart/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,neiljay/RIOT,lazytech-org/RIOT,x3ro/RIOT,ant9000/RIOT,mfrey/RIOT,biboc/RIOT,mfrey/RIOT,josephnoir/RIOT,BytesGalore/RIOT,roberthartung/RIOT,LudwigOrtmann/RIOT,OlegHahm/RIOT