How can I download NIOS2 image into a serial flash

For question "Could you help to suggest which boot loader file should be used?" , it depends on how to boot up the NIOS application. This document may help you out https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/nios2/edh_ed_handbook.pdf,table26. To convert programming file, refer to this link https://www.intel.cn/content/altera-www/global/zh_cn/index/documentation/suc1427659050061/suc1427659183639.html