Trusty Hello World in Rust

This TA reimplements the Quick Start Hello World TA under
go/trusty-docs in Rust.

Bug: None
Test: Downloading the Hello World directory, adding the Rust TA under
TRUSTY_BUILTIN_USER_TASKS, and running a boot test on qemu.

Change-Id: I837e0ca01c676f91153edc4403b8c45e037c12ba
8 files changed