Add "build.sh live-image" support for virt68k.

Briefly tested with qemu-system-m68k 9.1.2:
---
% qemu-system-m68k -machine virt \
 -drive file=NetBSD-10.99.12-virt68k-live.img,if=none,format=raw,id=disk \
 -device virtio-blk-device,drive=disk \
 -netdev user,id=net -device virtio-net-device,netdev=net \
 -kernel netbsd.virt68k -append root=ld0 -nographic
[   1.0000000] Initialized Goldfish TTY console @ 0xff008000
[   1.0000000] Memory segment 0: addr=0x00442000 size=0x07bbe000
[   1.0000000] entropy: ready
[   1.0000000] Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003,
[   1.0000000]     2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013,
[   1.0000000]     2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023,
[   1.0000000]     2024, 2025
[   1.0000000]     The NetBSD Foundation, Inc.  All rights reserved.
[   1.0000000] Copyright (c) 1982, 1986, 1989, 1991, 1993
[   1.0000000]     The Regents of the University of California.  All rights reserved.

[   1.0000000] NetBSD 10.99.12 (GENERIC) #1: Sun Jan 19 12:45:27 JST 2025
[   1.0000000] 	tsutsui@mirage:/s/cvs/src/sys/arch/virt68k/compile/GENERIC
[   1.0000000] Qemu 9.1.2: MC68040+MMU+FPU, 4k+4k on-chip physical I/D caches
[   1.0000000] total memory = 128 MB
[   1.0000000] avail memory = 121 MB
[   1.0000000] mainbus0 (root)

 :

[   1.2800030] root on ld0a dumps on ld0b
[   1.3000030] root file system type: ffs
[   1.3100030] kern.module.path=/stand/virt68k/10.99.12/modules
Sun Jan 19 04:55:45 UTC 2025
Starting root file system check:
/dev/rld0a: file system is clean; not checking

 :

Starting inetd.
Starting cron.
The following components reported failures:
    /etc/rc.d/postfix
See /var/run/rc.log for more information.
Sun Jan 19 04:56:06 UTC 2025

NetBSD/virt68k (Amnesiac) (constty)

login:
---

Author: tsutsui

distrib/virt68k/Makefile

@@ -1,11 +1,15 @@
-#	$NetBSD: Makefile,v 1.1 2024/01/07 00:57:13 thorpej Exp $
+#	$NetBSD: Makefile,v 1.2 2025/01/19 04:58:42 tsutsui Exp $
 
 .include <bsd.own.mk>
 
 SUBDIR=
 SUBDIR+=	ramdisks
 SUBDIR+=	.WAIT
+SUBDIR+=	liveimage
 
 TARGETS+=	release
 
+live_image:
+	${MAKEDIRTARGET} liveimage live_image
+
 .include <bsd.subdir.mk>

distrib/virt68k/liveimage/Makefile

@@ -0,0 +1,8 @@
+# $NetBSD: Makefile,v 1.1 2025/01/19 04:58:42 tsutsui Exp $
+
+SUBDIR=
+SUBDIR+=	emuimage
+
+TARGETS+=	release live_image
+
+.include <bsd.subdir.mk>

distrib/virt68k/liveimage/emuimage/Makefile

@@ -0,0 +1,12 @@
+#	$NetBSD: Makefile,v 1.1 2025/01/19 04:58:43 tsutsui Exp $
+
+LIVEIMGBASE=	NetBSD-${DISTRIBVER}-virt68k-live	# gives ${IMGBASE}.img
+
+.include <bsd.own.mk>
+
+KERN_SET=		kern-GENERIC
+.if defined(EMUIMAGEMB)
+LIVEIMAGEMB=		${EMUIMAGEMB}
+.endif
+
+.include "${.CURDIR}/../../../common/bootimage/Makefile.liveimage"