docheck: make doccheck executable from any directory
docheck: make doccheck executable from any directory

The doccheck script reports reports a false positive when executed from
any directory but `RIOTBASE`. With this fix, `make doc` changes into
the currently unused `RIOTBASE` variable.

This is an alternative approach to #7217, which removes this variable,
but keeps the false positive aspect of the script untouched.

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