rust: Simple main wrapper

Until further variants are needed, this provides a `main()` function
that simply calls into the `rust_main()` function.  This is adequate for
applications where main can be directly written in Rust.

The reason that this is written here, rather than just writing `main`
directly in Rust is that `kernel/main_weak.c` provides a weak default
implementation that will be brought in before the Rust application's
`main` is linked in.

Signed-off-by: David Brown <david.brown@linaro.org>