support/testing: add tests for Rust

To test the support for the Rust language, the following tests are added:

- building Rust compiler and Cargo from source.
- installing a pre-built Rust compiler and building Cargo from source.

For each test, a Rust test program is built and installed in the root file
system of a ARM vexpress QEMU system. The test is declared OK if the program can
be run properly from the test system.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>