Hello 文博 尚,
The uboot.imx is created based on the uboot.bin and the IVT header. There is more information on this thread: https://community.nxp.com/thread/309765
The uboot.bin file should be created as part of creating the uboot.imx file. It should be on the following path
<BSP_DIR>/<BUILD_DIR>/tmp/work/<MACHINE>-poky-linux-gnueabi/u-boot-imx/<u-boot_VERSION>/git
I hope this helps!
Regards,