flex-build -b sd -m ls1046ardb -i mkfw.
the *_sd.image should work,more info see the docs: https://www.nxp.com/docs/en/user-guide/LSDKUG_Rev21.08.pdf
If you are located in China and working on a non-verification project that has the potential for mass production, you can contact local distributors who can provide you with detailed design proposals and reference information.