makefiles/docker: export BOARDS
makefiles/docker: export BOARDS

This enables "BUILD_IN_DOCKER=1 BOARDS='foo bar' make buildtest".
Without this line, when executed with BUILD_IN_DOCKER, "make buildtest"
would always build all boards.

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