rust: correctly link rust-snapshot into build/stage0

rust: correctly link rust-snapshot into build/stage0

This does not seem to be used in regular builds, but is beneficial
in rust selftest, where it allows dropping a custom patch
that is unsuitable for upstream (and was rejected by them).

Also remove an obsolete comment that seems related to the code
but describes something that was resolved long time ago.

I have confirmed that the rust selftest continues to pass with just
this one commit on top of master (as the following changes do break
the selftest).

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>