tests: set default board=samr21-xpro for tests/periph_rtt
tests: set default board=samr21-xpro for tests/periph_rtt

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