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>

doc/README.ubi

@@ -14,6 +14,8 @@ ubi part [part] [offset]
 ubi info [l[ayout]] - Display volume and ubi layout information
 ubi create[vol] volume [size] [type] - create volume name with size
 ubi write[vol] address volume size - Write volume from address with size
+ubi write.part address volume size [fullsize]
+ - Write part of a volume from address
 ubi read[vol] address volume [size] - Read volume to address with size
 ubi remove[vol] volume - Remove volume
 [Legends]
@@ -77,6 +79,7 @@ ubi createvol	Create UBI volume on UBI device
 ubi removevol	Remove UBI volume from UBI device
 ubi read	Read data from UBI volume to memory
 ubi write	Write data from memory to UBI volume
+ubi write.part	Write data from memory to UBI volume, in parts
 
 
 Here a few examples on the usage: