pkg/pkg.mk: add checks for required variables
pkg/pkg.mk: add checks for required variables

Ensure the required variables are defined before including 'pkg.mk'.

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