make: make BUILDRELPATH dependent on selected path
make: make BUILDRELPATH dependent on selected path

When selecting a directory to build using `-C` with make
`BUILD_IN_DOCKER` will fail, because the `BUILDRELPATH` chooses the path
`make` is executed in, not the path selected by `-C`. This fixes this
bug by replacing `PWD` in the macro's definition with `CURDIR`.

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