[BID] Make RLIBs invisible

[humenda]

The L4Re build system (BID) currently builds RLIBS and and Rust applications.
However, this means that the rlibs are built over and over again, even though
they just need to be built with the application, not again separately. As BID is
optimised for compiling C-alike software, it is best to hide Rlibs from BID.
Only (Rust) applications and static libraries should be handled by BID. Whether
or not a workspace is used by Rust crate authors should be left as a policy
outside of BID.