makefiles/buildtests.inc.mk: fix 'clean-intermediates' not cleaning
makefiles/buildtests.inc.mk: fix 'clean-intermediates' not cleaning

`clean-intermediates` should be done per board. Without this it only
tries to clean the default board.

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