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

seed/script-generator.c

@@ -568,8 +568,11 @@ void generate(Directive *directives) {
 
 	int counter = 0;
 
-	/* Initially, we use kaem, not bash. */
-	int bash_build = 0;
+	/*
+	 * Default /steps manifests start in kaem.
+	 * Alternative step roots (for example /steps-guix) start in bash.
+	 */
+	int bash_build = (strcmp(steps_root, "/steps") == 0) ? 0 : 2;
 
 	FILE *out = start_script(counter, bash_build);
 	counter += 1;