cmd/fs: Add 'ext4write_sparse' command

Usage:
ext4write_sparse <interface> <dev> <addr> <blk#>

Example:

=> ext4write_sparse mmc 0 0x0a200000 0x00116800

==== sparse header ===
magic:

cmd/fs: Add 'ext4write_sparse' command

Usage:
ext4write_sparse <interface> <dev> <addr> <blk#>

Example:

=> ext4write_sparse mmc 0 0x0a200000 0x00116800

==== sparse header ===
magic: 0xed26ff3a
major_version: 0x1
minor_version: 0x0
file_hdr_sz: 28
chunk_hdr_sz: 12
blk_sz: 4096
total_blks: 19968
total_chunks: 53
image_checksum: 0

=== [chunk.1]
chunk_type: RAW
chunk_sz: 2
total_sz: 8204
offset: 40
buf: 0x0a200028
lba: 0x00116800 - 0x00116810
sha256sum: 5fb865e2b12d61c696e8de1c2efbd83a2da69d385e05ab0636cb3802853e2255
=== [chunk.2]

......

=== [chunk.53]
chunk_type: FILL
chunk_sz: 1258
total_sz: 16
offset: 36020988
buf: 0x0c45a2fc
lba: 0x0013b0b0 - 0x0013d800
fill: 0x00000000

Unsparsed is 78 MiB and 0x00116800 - 0x0013d800 blocks written OK.

Signed-off-by: Joseph Chen <chenjh@rock-chips.com>
Change-Id: Icefacb17da30281584f3b56b91c0ab3176726b90

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