This module creates ext4 SD card images from the build output. These
images contain partition tables and 2 partitions.

These images are in the "ZD" format, suitable for flashing via OpenFirmware's
fs-update command. Corresponding .zsp files are produced too. To disable
.zd/.zsp creation, set the make_zd option to 0 (default is 1).

By default, a single disk image is produced, with it's size automatically
calculated based on the size of the data included in the image, plus a small
amount of free space. The system will resize the partition and filesystem to
fill the media during first boot. Therefore, other than loading the module,
no configuration is usually necessary.

If you wish to create multiple images, or if you wish to specify a size,
you can specify some uniquely-named options starting with "size_". (This
will disable the default creation of an automatic-sized image as described
above.)

The value for each of these options is the disk size, optionally followed by
a comma and a custom extension. There can only be one un-named entry, which
will default to the extension "zd". The disk size is specified in bytes,
but the special size "auto" will result in the disk size being calculated
automatically as described above.

For example:
[sd_card_image]
; 3780mb
size_1=3964665000
; 1985mb
size_2=1977614336,zd2

The output files will be:

	(the 3780mb disk output)
	osXX.zd
	osXX.zsp

	(the 1985mb disk output)
	osXX.zd2
	osXX.zd2.zsp

A "keep_disk_image" option is available, which can be used to retain the
raw disk image file (basically a file that looks like a block device) that was
used to produce the ZD image, which you could flash with Linux using the "dd"
command.

For example:

[sd_card_image]
keep_disk_image=1
size_1=3964665000

The output files will be:

	osXX.zd
	osXX.zsp
	osXX.zd.disk.img.tar.gz

The raw image file is archived in tar (in sparse mode) and then compressed with
gzip. Tar preserves the sparseness of the file (the fact that some part of the
image is made up of unallocated disk blocks). To disable this
archiving/compression, set the compress_disk_image option to 0 (default is 1).
