bootstrap: add stage0 image reuse for guix and simplify guix handoff flow

- add --stage0-image in rootfs.py for qemu to boot an existing kernel-bootstrap image
- when --stage0-image is combined with --build-guix-also, update BUILD_GUIX_ALSO in image config and sync /steps-guix into the image
- require stage0 /init to contain guix handoff marker instead of patching /init implicitly
- add run_steps_guix_if_requested() to make_bootable-generated /init so rebooted stage0 images can enter steps-guix directly
- run /steps-guix/0.sh with bash in after.sh
- make script-generator start mode convention-based: /steps stays kaem-first, alternate roots (e.g. /steps-guix) start in bash
- remove redundant explicit /steps config-root argument from seed/preseeded/reconfigure script-generator calls

steps/improve/after.sh

@@ -27,7 +27,7 @@ if [ "${BUILD_GUIX_ALSO}" = True ]; then
     echo 'BUILD_GUIX_ALSO=False' >> /steps/bootstrap.cfg
 
     /script-generator /steps-guix/manifest /steps
-    kaem --file /steps-guix/0.sh
+    bash /steps-guix/0.sh
 fi
 
 if [ "${INTERACTIVE}" = True ]; then