Allow Resize to a defined size at first boot, not only to the maximum of the storage device

[hrpv]

Maybe this change is interesting for other people too
https://github.com/hrpv/RonR-RPi-image-utils/blob/master/resize-root-fs/resize.md

the modified code of resize-root-fs is not integrated in the image-backup script.

Thank you and RonR the original for providing this repository of really useful image-utils.