Update library axtls and remove sources from our repo

[flsobral]

Currently we use the version 1.5.4, customized for usage with the tcvm. The objective is to update to version 2.1.5 (the latest available at this moment), remove the sources from our repo, create patches with our changes and submit them back to the original project.

• [ ] Update the sources in our repo with version 2.1.5 and our changes (to have TLS 1.2 support as soon as possible)
• [ ] Convert these changes into patches
• [ ] Download, patch and build axtls automatically as a static library using CMake
• [ ] Remove axtls sources from our repo
• Test new build on every target
  • [ ] Linux x64
  • [ ] Linux arm32v7
  • [ ] Linux arm64
  • [ ] Android
  • [ ] iOS
  • [ ] Win32
  • [ ] WinCE
  • [ ] MacOS (only used for debugging, but that's important for development)
• [ ] Submit patches back to the original project