Allow to build encrypted images via jenkins
This enables building encrypted images in jenkins by adding a luks option to the jenkins builds (on top of the other changes for local building merged earlier). The job name of encryted builds starts with luks instead of plain so the librem5-flash-image-script doesn't confuse these. Once this is in we can also add an option to librem5-flash-image to flash encrypted right away.
Once merged the encrypted builds will fail until osk-sdl migrated from landing to byzantium due to /boot running out of space in the installer (doesn't affect already installed systems since those get smaller initramfs anyway).
We'll see how well jenkins copes with the 4G images. If it has issues we can push out to storage.