[yocto] Support UBI u-boot

[JH]

Changed to https://source.codeaurora.org/external/imx/uboot-imx.git
branch imx_v2018.03_4.14.98_2.0.0_ga, it can now support
CONFIG_CMD_MTDPARTS ind defconfg, all MTDPARTS error gone, but then
2018.03-r0/git/cmd/ubi.c missed dependencies of del_mtd_partitions,
add_mtd_partitions, mtd_get_device_size all in mtdpart.c which can
only be enabled by CONFIG_MTD_PARTITIONS:

obj-$(CONFIG_MTD_PARTITIONS) += mtdpart.o

But that CONFIG_MTD_PARTITIONS is not supported in defconfig.

Could anyone advise which branch works for UBI?

Thank you.

Kind regards,

- jh