Hi Wang,
Thank you for your reply. I think there is some misunderstanding, I'm not trying to boot from sd-card, I'm trying to upgrade rootfs, kernel(zImage) or device tree(.dtb) from sd-card(a storage device, not bootable) in u-boot. Because of there will be only one way to upgrade the end-product, sd-card. In u-boot, I can see my files which are saved in sd-card by using command "ls mmc":
=> ls mmc 0:1
<DIR> 4096 .
<DIR> 4096 ..
<DIR> 16384 lost+found
398006093 fsl-image-qt5-imx6dlsabresd-20170829113635.rootfs.tar.bz2
6577200 zImage
So, how do I upgrade rootfs, kernel(zImage) or device tree(.dtb) to eMMC in u-boot from SD-card?