Fix flatpak in bootable images
Flatpak session helper requires to have a flatpak user and fuse to work. Fuse was missing setuid on fusermount.
When polkit will stop depending on mozjs, it is likely we will just move it to freedesktop-sdk, including flatpak. For the moment the version of flatpak that is in the freedesktop-sdk does not have the session helper. It is used more as a development tool.
Fuse counterpart element in has been fixed. Maybe we should think of using fuse from freedesktop-sdk.