Function build_dhcp_payload 

fn build_dhcp_payload(
    msg_type: u8,
    xid: u32,
    mac: [u8; 6],
    yiaddr: [u8; 4],
    req_ip: [u8; 4],
    server: [u8; 4],
    lease_secs: u32,
    out: &mut [u8],
) -> Result<usize, &'static str>