Calculating system image size does not ignore home folder

Created by: ollieparanoid

@opendata26 reported in IRC:

the system image size also includes things in the home folder [in the rootfs chroot]

The thing is, that we do not copy this folder to the actual image (and this isn't really documented anywhere, which makes it totally understandable that we didn't pay attention to this), so the size calculation is slightly wrong, when there's a big file in the home folder.

@drebrez: Since you have implemented the minimum size calculation, would you like to take a look at this?