tools/pr_check: make sure no pkg makefile defines PKG_SOURCE_LOCAL
tools/pr_check: make sure no pkg makefile defines PKG_SOURCE_LOCAL

The variable is supposed to be used for local development only.

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