* guix/scripts/system.scm (list-image-types): New procedure,
(%options): add "image-type" and "list-image-types" options, remove
"file-system-type" option,
(show-help): adapt accordingly,
(%default-options): also adapt, and set the default "image-type" to "raw",
(perform-action): add image-type argument and remove file-system-type argument,
(process-action): adapt perform-action call,
(system-derivation-for-action): remove base-image
argument, add image-type argument, and use it to create the image passed to
"system-image".
* tests/guix-system.sh: Adapt accordingly and add a test for
"--list-image-types" command.
* doc/guix.texi (Building the Installation Image,
Invoking guix system): Adapt accordingly.
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>