It takes some time to write zeros to the disk and it aborts later if
less than 20GB are available.
Add variable to the template, cleanup script and add a new option to
wrapacker.
The '==>' prefix is used by pacstrap, pacman, and mkinitcpio. A new
prefix is needed to better differentiate the output of the script from
the output of the programs mentioned above.
Also adding more output to the different steps helps during troubleshooting.
Do a final clean of the pacman cache, then write zeros to disk and
unlink the file. Packer will compact the disk as part of the build,
resulting in a smaller final image.