Function prepare_boot_config 

pub async fn prepare_boot_config(
    infra: &InfraContext<'_>,
    token: &str,
    hosts_expression: &str,
    new_boot_image_id_opt: Option<&str>,
    new_boot_image_configuration_opt: Option<&str>,
    new_kernel_parameters_opt: Option<&str>,
) -> Result<BootConfigChangeset, Error>

Prepare boot configuration changes (no side effects).