Replace license.txt contents with Apache 2.0 license
Replace license.txt contents with Apache 2.0 license

License text taken from: https://www.apache.org/licenses/LICENSE-2.0.txt

KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS,KhronosGroup/OpenCL-CTS