Integrate xconfig/add_LICENSE.txt

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Commit: seco-ne/tools/xconfig@28791fe4

LICENSE: Add Apache 2.0 license file

According to our company licensing policy for software the Yocto layers
should be licensed under the Apache 2.0 license.
Adapt the LICENSE-2.0.txt file from https://www.apache.org/licenses/LICENSE-2.0.txt
as described in the APPENDIX.

Add the license as LICENSE.txt file to the layer repository.

BCS 746-000657

Change-Id: I247b2eb004aea29a13059d6e64bb0bc3d06ee9fe