makefiles/vars.inc.mk: document BOARDSDIR as external boards directory
makefiles/vars.inc.mk: document BOARDSDIR as external boards directory

Replace using 'RIOTBOARD' by 'BOARDSDIR' to define external boards.

OlegHahm/RIOT,yogo1212/RIOT,josephnoir/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,mtausig/RIOT,basilfx/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,x3ro/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,mtausig/RIOT,toonst/RIOT,OTAkeys/RIOT,basilfx/RIOT,yogo1212/RIOT,ant9000/RIOT,josephnoir/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,smlng/RIOT,kaspar030/RIOT,OTAkeys/RIOT,toonst/RIOT,OlegHahm/RIOT,miri64/RIOT,josephnoir/RIOT,kaspar030/RIOT,ant9000/RIOT,josephnoir/RIOT,smlng/RIOT,smlng/RIOT,kaspar030/RIOT,yogo1212/RIOT,basilfx/RIOT,authmillenon/RIOT,jasonatran/RIOT,x3ro/RIOT,OTAkeys/RIOT,x3ro/RIOT,mtausig/RIOT,kYc0o/RIOT,basilfx/RIOT,toonst/RIOT,miri64/RIOT,x3ro/RIOT,authmillenon/RIOT,authmillenon/RIOT,toonst/RIOT,miri64/RIOT,x3ro/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kaspar030/RIOT,yogo1212/RIOT,toonst/RIOT,yogo1212/RIOT,miri64/RIOT,yogo1212/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,mtausig/RIOT,mtausig/RIOT,smlng/RIOT,jasonatran/RIOT,OTAkeys/RIOT