cmd_ubi: add write.part command, to write a volume in multiple parts

This allows you to write data to an UBI volume when the amount of memory
available to write that data from is less than the total size of the
data. For example, you may split a root filesystem UBIFS image into
parts, provide the total size of the image to the first write.part
command and then use multiple write.part commands to write the
subsequent parts of the volume. This results in a sequence of commands
akin to:

  ext4load mmc 0:1 0x80000000 rootfs.ubifs.0
  ubi write.part 0x80000000 root 0x08000000 0x18000000
  ext4load mmc 0:1 0x80000000 rootfs.ubifs.1
  ubi write.part 0x80000000 root 0x08000000
  ext4load mmc 0:1 0x80000000 rootfs.ubifs.2
  ubi write.part 0x80000000 root 0x08000000

This would write 384MiB of data to the UBI volume 'root' whilst only
requiring 128MiB of said data to be held in memory at a time.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Stefan Roese <sr@denx.de>