FirmwareInstall method could wrap io.Reader for file length

[joelrebel]

If length is required, maybe consider a different interface like fs.File or you could declare an interface like

type LengthReader interface {  
    io.Reader
    Len() int
}

which is satisfied by bytes.Buffer and strings.Reader and provide a helper for os.File/fs.File like

type lr struct {
  fs.File
}

func (lr *r) Len() int {
    info, err := r.Stat()
    if err != nil {
        panic(err) // or ignore/log
    }
    return int(info.Size())
}

// LengthReaderFromFile returns a LengthReader from an fs.File
func LengthReaderFromFile(f fs.File) LengthReader {
    return &lr{f}
}

Originally posted by @micahhausler in #261 (comment)