add support for zerofree the partitions

[rgl]

it would be nice to be able to call zerofree on the rootfs device before detaching the loop device.

doing that makes the img more compressible when we do a apt-get dist-upgrade while provision the image.

for example, after zerofree the image manually, the zip file went from 826306919 to 509377581 bytes.

BTW, this is also done on the upstream raspbian images.