It is documented there:
So when booted from SD you need to copy over the u-boot image to SD card and do from there:
dd if=cubie_a5e_spi_nor_raw_050624.img of=/dev/mtdblock0
But that won't you get NVMe support on Armbian as Cubie A5E NVMe support just isn't in the kernel, not even the dev version.