Add optional writable partition

When an Archiso-generated ISO is put on an USB drive the non-rockridge partitions (i.e. the EFI system partition) become writable.

This merge request adds support for a third native (non-fat) partition which can be added to the ISO file to be used as persistent storage at build time.

persistent_image_type:   ext4
                         ext4+luks
persistent_size:         size in KB

The partition is added to the ISO whenever persistent_size is set or a non-empty persistent directory is found in the profile directory. When selecting the +luks image type, the same encryption key as the airootfs is used.

When it's entirely unfeasible to ship an enlarged ISOs, a systemd service (setup-persistent-storage), which depends on xorriso being present in the live environment and executes the same task described above on first boot, has been added to the baseline and releng profiles.

If I were to estimate proper sizes for current profiles, they would be around 10MB and 50MB, so it would mostly be to let the system import and edit systemd-homed homes on internal/external drives.

Solves #184 (closed), partially #171 (closed).

Depends on !217 (closed), !253 (closed), !259 (closed).