scripts: add `rust_is_available.sh`

This script tests whether the Rust toolchain requirements are in place
to enable Rust support. It uses `min-tool-version.sh` to fetch
the version numbers.

The build system will call it to set `CONFIG_RUST_IS_AVAILABLE` in
a later patch.

It also has an option (`-v`) to explain what is missing, which is
useful to set up the development environment. This is used via
the `make rustavailable` target added in a later patch.

Reviewed-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Co-developed-by: Finn Behrens <me@kloenk.de>
Signed-off-by: Finn Behrens <me@kloenk.de>
Co-developed-by: Miguel Cano <macanroj@gmail.com>
Signed-off-by: Miguel Cano <macanroj@gmail.com>
Co-developed-by: Tiago Lam <tiagolam@gmail.com>
Signed-off-by: Tiago Lam <tiagolam@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>