Rollup merge of #136615 - Ayush1325:uefi-net-unsupported, r=joboet

sys: net: Add UEFI stubs

- Just a copy of sys/net/unsupported.
- Will make the future net PRs easier to review.
- The reason for a separate folder instead of standalone file is that UEFI has separate the protocols for v4 and v6, and thus will need some abstractions to implement the Rust interface.

r? ``@jhpratt``

Author: matthiaskrgr