Rust-lang (rustc/cargo) for OpenWrt - testing needed

[Grommish]

Rustup thread.

[rustup] Rust-lang package manager

There is an existing rust-lang package that I worked on that was completed after I went on sabbatical. I had issues prior to leaving with massive compile times and issues when switching targets. While gone, I decided to look into using rustup instead when I got back. This wasn't an option for me originally as I didn't know enough about either the Openwrt build system or rust-lang in general. This is what rustup currently solves. Rustup does NOT require a rebuild between targets, since it do…