Makefile.include: only use board include directory if it exists
Makefile.include: only use board include directory if it exists

Some boards do not have a boards/BOARD/include directory and rely on a
board/common one.

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