Grow system image if needed

[UniversalSuperBox]

On devices with a system image, we should make ubports-qa attempt to make the system-image larger.

Ideally, this will be done from recovery using some type of system-image command.

[nfsprodriver]

https://github.com/plasma-mobile/plasma-phone-dev-setup/blob/master/usr/bin/resize-root-partition
As workaround, that would be a good thing. But when I remember right this get's reverted by a system update or even makes system updates impossible.

[UniversalSuperBox]

#14