Automatically expand root filesystem

[simonvanderveldt]

We'll need to figure out how to expand the rootfs on first boot.
This is especially relevant for the Raspberry Pi images, but might also be relevant for the image for norns since that could run on a CM3+ with more storage.
Alternatively we could create several images for norns with CM3 or CM3+ with smaller/larger root filesystem sizes.

Despite it being pretty obvious basic functionality I haven't been able to find an out of the box option for buildroot.
Some sources for inspiration could https://source.puri.sm/Librem5/librem5-devkit-tools/merge_requests/37/diffs and https://github.com/asb/raspi-config/blob/master/raspi-config#L36

[simonvanderveldt]

Depending on the choice of update tool (see #17) this might be something we can configure in the update tool, see for example https://github.com/fhunleth/fwup/blob/master/README.md#mbr