rust: Switch to use RUST_XXX_SYS consistently

rust: Switch to use RUST_XXX_SYS consistently

The code was using a mixture of XXX_SYS and RUST_XXX_SYS. Use
RUST_XXX_SYS consistently and add the variables to the global exclsion
on signatures as they're reflected in the directory triplets and trying
to filter them out the hashes separately is too painful.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>