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-guix/binutils-2.41/patches/new-gettext.patch

@@ -0,0 +1,17 @@
+SPDX-FileCopyrightText: 2023 Samuel Tyler <samuel@samuelt.me>
+
+SPDX-License-Identifier: GPL-3.0-or-later
+
+In new gettext external is required for AM_GNU_GETTEXT.
+
+--- binutils-2.41/intl/configure.ac	2023-02-07 18:57:56.350832016 +1100
++++ binutils-2.41/intl/configure.ac	2023-02-07 18:58:07.310054484 +1100
+@@ -4,7 +4,7 @@
+ AC_CONFIG_HEADER(config.h)
+ AC_CONFIG_MACRO_DIR(../config)
+ AM_GNU_GETTEXT_VERSION(0.12.1)
+-AM_GNU_GETTEXT([], [need-ngettext])
++AM_GNU_GETTEXT([external], [need-ngettext])
+ 
+ # This replaces the extensive use of DEFS in the original Makefile.in.
+ AC_DEFINE(IN_LIBINTL, 1, [Define because this is libintl.])