cpu/samd21: skip pwm if not defined in board config
cpu/samd21: skip pwm if not defined in board config

OTAkeys/RIOT,LudwigOrtmann/RIOT,ks156/RIOT,ks156/RIOT,BytesGalore/RIOT,yogo1212/RIOT,basilfx/RIOT,A-Paul/RIOT,ant9000/RIOT,authmillenon/RIOT,ks156/RIOT,hamilton-mote/RIOT-OS,lazytech-org/RIOT,cladmi/RIOT,ks156/RIOT,OTAkeys/RIOT,adrianghc/RIOT,A-Paul/RIOT,immesys/RiSyn,jasonatran/RIOT,A-Paul/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,x3ro/RIOT,neiljay/RIOT,miri64/RIOT,immesys/RiSyn,toonst/RIOT,miri64/RIOT,biboc/RIOT,Josar/RIOT,gebart/RIOT,adrianghc/RIOT,beurdouche/RIOT,A-Paul/RIOT,smlng/RIOT,immesys/RiSyn,rfuentess/RIOT,BytesGalore/RIOT,mfrey/RIOT,RIOT-OS/RIOT,neiljay/RIOT,LudwigKnuepfer/RIOT,miri64/RIOT,BytesGalore/RIOT,adrianghc/RIOT,LudwigOrtmann/RIOT,smlng/RIOT,josephnoir/RIOT,cladmi/RIOT,OTAkeys/RIOT,kbumsik/RIOT,OTAkeys/RIOT,kbumsik/RIOT,aeneby/RIOT,aeneby/RIOT,LudwigKnuepfer/RIOT,kaspar030/RIOT,roberthartung/RIOT,mtausig/RIOT,LudwigOrtmann/RIOT,smlng/RIOT,kbumsik/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,yogo1212/RIOT,Josar/RIOT,cladmi/RIOT,kaspar030/RIOT,josephnoir/RIOT,neiljay/RIOT,authmillenon/RIOT,roberthartung/RIOT,immesys/RiSyn,LudwigKnuepfer/RIOT,ks156/RIOT,RIOT-OS/RIOT,LudwigOrtmann/RIOT,beurdouche/RIOT,cladmi/RIOT,LudwigOrtmann/RIOT,neiljay/RIOT,beurdouche/RIOT,aeneby/RIOT,toonst/RIOT,mfrey/RIOT,LudwigKnuepfer/RIOT,ant9000/RIOT,yogo1212/RIOT,ant9000/RIOT,x3ro/RIOT,A-Paul/RIOT,roberthartung/RIOT,aeneby/RIOT,x3ro/RIOT,cladmi/RIOT,kaspar030/RIOT,adrianghc/RIOT,gebart/RIOT,LudwigOrtmann/RIOT,kYc0o/RIOT,OTAkeys/RIOT,Josar/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,avmelnikoff/RIOT,gebart/RIOT,gebart/RIOT,Josar/RIOT,hamilton-mote/RIOT-OS,lazytech-org/RIOT,authmillenon/RIOT,kYc0o/RIOT,Josar/RIOT,neiljay/RIOT,hamilton-mote/RIOT-OS,OlegHahm/RIOT,RIOT-OS/RIOT,smlng/RIOT,mtausig/RIOT,toonst/RIOT,beurdouche/RIOT,josephnoir/RIOT,beurdouche/RIOT,authmillenon/RIOT,rfuentess/RIOT,basilfx/RIOT,toonst/RIOT,rfuentess/RIOT,jasonatran/RIOT,josephnoir/RIOT,lazytech-org/RIOT,hamilton-mote/RIOT-OS,rfuentess/RIOT,immesys/RiSyn,RIOT-OS/RIOT,RIOT-OS/RIOT,aeneby/RIOT,BytesGalore/RIOT,adrianghc/RIOT,mtausig/RIOT,josephnoir/RIOT,roberthartung/RIOT,LudwigKnuepfer/RIOT,roberthartung/RIOT,biboc/RIOT,biboc/RIOT,BytesGalore/RIOT,yogo1212/RIOT,kaspar030/RIOT,x3ro/RIOT,jasonatran/RIOT,kbumsik/RIOT,mtausig/RIOT,ant9000/RIOT,immesys/RiSyn,toonst/RIOT,basilfx/RIOT,lazytech-org/RIOT,mfrey/RIOT,hamilton-mote/RIOT-OS,mtausig/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,OlegHahm/RIOT,mfrey/RIOT,kbumsik/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,x3ro/RIOT,avmelnikoff/RIOT,biboc/RIOT,basilfx/RIOT,jasonatran/RIOT,mfrey/RIOT,miri64/RIOT,kYc0o/RIOT,basilfx/RIOT,authmillenon/RIOT,rfuentess/RIOT,lazytech-org/RIOT,gebart/RIOT,kYc0o/RIOT,smlng/RIOT,avmelnikoff/RIOT,biboc/RIOT