Makefile.include: Fix BUILDRELPATH when RIOTPROJECT is CURDIR.
Makefile.include: Fix BUILDRELPATH when RIOTPROJECT is CURDIR.

This fixes building an example that is outside of RIOT with docker and
also fixes building from a release archive.

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