Rust nightly requires -Zunstable-options for custom target JSON specs
(such as x86-unknown-none.json). Without it, the build fails with:
error: error loading target specification: custom targets are
unstable and require `-Zunstable-options`
UEFI already uses the built-in x86_64-unknown-uefi target, so it
didn't need this flag. The kernel recipe already has this fix; the
bootloader BIOS Makefile was missing it.
Red Bear OS