pkg/openthread: define PKG_LICENSE to BSD 3-Clause
pkg/openthread: define PKG_LICENSE to BSD 3-Clause

The 'BSD 3-Clause "New" or "Revised" License' is the value found by
github

https://github.com/openthread/openthread/blob/3a248f649acd16448c7f27fe7f17f6f0bb6696f6/LICENSE

> A permissive license similar to the BSD 2-Clause License, but with a 3rd
> clause that prohibits others from using the name of the project or its
> contributors to promote derived products without written consent.

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