Add mention to tinycrypt LICENSE in main LICENSE

Signed-off-by: Fabio Utzig <utzig@apache.org>

LICENSE

@@ -199,3 +199,8 @@
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
+
+This product bundles tinycrypt, which is available under the "3-clause BSD"
+license. For details, and bundled files see:
+    * ext/tinycrypt/LICENSE
+    * ext/tinycrypt