Hello Dana Xiong,
You could generate SD boot RCW with rcw package.
$ flex-builder -c rcw -m ls1043ardb
Please modify packages/firmware/rcw/ls1043ardb/RR_FQPP_1455/rcw_*_sdboot.rcw according to your requirement.
Then run "flex-builder -c rcw -m ls1043ardb" again, and get packages/firmware/rcw/ls1043ardb/RR_FQPP_1455/rcw_*_sdboot.bin
Please run the command "xxd rcw_*_sdboot.bin" to display rcw binary file.
Then modify packages/firmware/u-boot/board/freescale/ls1043ardb/ls1043ardb_rcw_sd.cfg according to rcw_*_sdboot.bin
Thanks,
Yiping