ext/mbedtls-asn1: Add support for mbedtls sha512.c

[rajivr]

sha512.c is needed by curve22519.c. By having sha512.c support in ext/mbedtls-asn1, we can do away with the requirement of having to import mbedtls into our tree just for sha512.c support.

As per PR #567, the imported mbedtls files are from mbedtls LTS version 2.16.3.

[utzig]

we can do away with the requirement of having to import mbedtls into our tree just for sha512.c support.

Are you using tinycrypt for the sha256? This does not include any changes to build scripts, which means sha512.c will only stay there without use (for most users); or are you planning on also adding the build changes?

[rajivr]

Are you using tinycrypt for the sha256?

Yes, I think so. I am still working on the port and here is how our mcuboot_config.h looks.

This does not include any changes to build scripts, which means sha512.c will only stay there without use (for most users); or are you planning on also adding the build changes?

I was not planning on making any changes to the build scripts. However, if there is interest upstream in adding enc_ec256 + sig_ed25519 configuration to build.rs without mbedtls, I can submit another PR enabling that configuration.

We are using Rust as well so I am somewhat familiar with build.rs, but not the rest of the simulator code

[rajivr]

Closing in favor of #627