Add gzwrite command to write gzip-compressed images to block devices.

Input must be gzip-compressed according to RFC1952, since the crc
and file size in the trailer will be confirmed during operation.
The decompressed file size must be specified on the command line
for images with decompressed sizes >= 4GiB because the trailer
only contains the low 32 bits of the original file size.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>