axtls for Unikraft

This is the port of axtls for Unikraft as external library. You will need lwip to make it work.

To run ssltest.c as a unit test, you'll need pthread support, e.g., add the following line to the LIBS variable in the Makefile:

...$(UK_LIBS)/lwip:$(UK_LIBS)/pthread-embedded: $(UK_LIBS)/newlib:$(UK_LIBS)/axtls...

Note that you'll need a filesystem in place and the certificate files for all tests to pass.

Please refer to the README.md as well as the documentation in the doc/ subdirectory of the main unikraft repository.