TLS, curl update, make tests run again

[JanneKiiskila]

Description

1. Update Mbed TLS version to 2.28.0 to cover for https://github.com/advisories/GHSA-7g56-f7p4-fmcq.
   • Ref: https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2021-12.
2. Also, curl has plenty of CVEs, see https://curl.se/docs/vuln-7.82.0.html.
3. The tests didn't run - README.md was not up-to-date and and curl include was not in place.

OK (160 tests, 160 ran, 18293 checks, 0 ignored, 0 filtered out, 169 ms)

[ERR ][edge-io]: Cannot remove the socket lock file: /tmp/foo.lock
[ERR ][edge-io]: Cannot unlock the socket lock: /tmp/foo.lock
[ERR ][edge-io]: The socket lock '/tmp/foo.lock' is held by another process. Is there another Edge Core running?
[ERR ][edge-io]: The socket lock file '/tmp/foo.lock' cannot be created. Please check the permissions!
...
OK (8 tests, 8 ran, 31 checks, 0 ignored, 0 filtered out, 1 ms)

Test instructions

Submitter: Please add instructions to test your PR here.

Check list

API change(s)

• [x] Not applicable.
• [ ] API is backwards compatible.
• [ ] API documentation is updated.

Example applications updated

• [x] Not applicable.

  Changelog

• [x] CHANGELOG.md updated.

[JanneKiiskila]

https://github.com/PelionIoT/mbed-edge/pull/87 supercedes this, closing.