system: image: Remove "image-root" when building raw disk-images.

system: image: Remove "image-root" when building raw disk-images.

The "image-root" derivation output is used as a temporary directory that is
passed to mke2fs and mkdosfs later on. By merging the creation of this
directory and the production of partition images, we can get rid of the
derivation.

As mke2fs and mkdosfs are not able to override file permissions, call those
commands with fakeroot. This way, all the image files will be owned by root,
even if image generation is done in an unprivilegded context.

* gnu/system/image.scm (system-disk-image): Merge "image-root" and
"iso9660-image" derivations so that we spare an extra derivation. Also add
"fakeroot" and its runtime dependencies to the inputs.
* gnu/build/image.scm (make-ext-image, make-vfat-image): Make sure that mke2fs
and mkdosfs are respectively called by fakeroot.