makefiles/vars.inc.mk: UNDEF fix variable doc
makefiles/vars.inc.mk: UNDEF fix variable doc

Despite its name, UNDEF are files that must be included during linking even if
the linker would not by default.

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