Fix permissions for files installed by image-configuration class

Currently the configuration files installed directly to the root filesystem are owned by the user that runs the build. This causes pollution of the target rootfs from the build host and problems because of unsuitable file ownership and permissions.

To solve this use a fakeroot environment and make the files owned by root by default.

Also implement a task that uses the permission fixup implementation from the package class to apply the same permission and ownership rules as when generating packages.