[coreboot] proposal for cross compilation: add Rust support

[ron minnich]

https://review.coreboot.org/c/coreboot/+/74124

First step, which I almost certainly did incorrectly, is to add Rust
toolchain support to the Makefile.

Next, adding a rust payload was suggested. I'll look into that, probably
rewriting linuxcheck in rust (since I'm the only user anyway :-)