I built imx8mpevk board, can find the uboot folder and mkimage folder under flexbuild/build_lsdk2512/bsp, for bld -m <machine>, you can get the BSP firmware image which consists
of the ATF, U-Boot, OP-TEE OS, kernel, dtb, peripheral firmware, and initramfs under flexbuild/build_lsdk2512/images, if you only need uboot, you can use the command "bld uboot -m <machine>"