Bootloaders can opt-in to the new functionality by setting a `package.metadata.bootloader.build-std` key with a comma-separated string of sysroot crates that should be built. For example, a setting of `build-std = "core"` results in the build command `cargo build -Zbuild-std=core`.

If no such key is set, bootimage continues to compile the bootloader using the command `cargo xbuild`.