Add support for bundling Openssl/LibreSSL/BoringSSL

[colmmacc]

Now that we're going to depend on Openssl 1.0.2, it would be nice to have clear instructions on how to build OpenSSL, or LibreSSL, or BoringSSL locally and to use those within s2n. I think we should be able to support each, it looks like they all have the APIs we're using. Additionally, we should consider using libcrypto.a - ie statically linking in libcrypto.

[colmmacc]

This issue has now been addressed in the USAGE-GUIDE