tools/buildsystem_check: exclude tests/external_board_native path
tools/buildsystem_check: exclude tests/external_board_native path

This test application must act as a regular application to showcase how to provide a board support from an external directory

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