Please get u-boot source code with the following command.
$ bitbake u-boot -c patch -f
Go to u-boot source code folder build_<board>/tmp/work/<board>-fsl-linux/u-boot-qoriq/<version>+fslgit-r0/git/, please eidt configs/<board>_defconfig file add the following configuration.
CONFIG_CMD_DHCP=y
Go back to folder build_<board>, rebuild u-boot with the following command.
$ bitbake u-boot