The Mbed TLS project no longer requires a CLA. Contributions from now on
must be made under both Apache-2.0 AND GPL-2.0-or-later licenses, to enable
LTS (Long Term Support) branches of the software to continue to be provided
under either Apache-2.0 OR GPL-2.0-or-later. Contributors must accept the
terms of the Developer Certificate of Origin (DCO) by adding a Signed-off-by:
line to each commit message.

The software on the development branch continues to be provided under
Apache-2.0.

Update README.md, CONTRIBUTING.md and pull_request_template.md to explain
the new licensing model. Add a copy of the DCO to the project.

Expand the full Apache-2.0 license text in the LICENSE file and remove the
redundant apache-2.0.txt.

Signed-off-by: Dan Handley <dan.handley@arm.com>