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.

pkg/openthread/Makefile

@@ -1,6 +1,7 @@
 PKG_NAME=openthread
 PKG_URL=https://github.com/openthread/openthread.git
 PKG_VERSION=thread-reference-20180926
+PKG_LICENSE=BSD-3-Clause
 PKG_BUILDDIR ?= $(PKGDIRBASE)/$(PKG_NAME)
 
 ifneq (,$(filter openthread-ftd,$(USEMODULE)))