image: Rename "raw" image-type to "efi-raw".

* gnu/system/image.scm (raw-image-type): Rename to "efi-raw-image-type".
* guix/scripts/system.scm (%default-options): Adapt accordingly.
* doc/guix.texi: Ditto.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>