修改启动顺序需要进入uboot界面，可以参考下IOT2050 github上的说明

Selecting a boot device

By default, the boot loader will pick the first bootable device. If that device may no longer fully start, you can select an alternative boot device in the U-Boot shell. Attach a USB-UART adapter to X14, connect it to a host PC and open a terminal program on that port. Reset the device and interrupt the boot when it counts down ("Hit any key to stop autoboot"). Then type

=> setenv boot_targets mmc0
=> run distro_bootcmd

to boot from the microSD card. Use usb0 for the first USB mass storage device.

NOTE: This selection is not persistent. The boot loader will fall back to its default boot order after reset.