distrib/luna68k: add "build.sh live-image" support for luna68k

Using recently added "USE_MDBOOTPART" in Makefile.bootimage with MD rules.
A generated live-image is confirmed working on nono 1.6.4 with LUNA's ROM:
---
% nono-cli -V vmtype=luna -V prom-image=LUNAROM.DAT -V dipsw-autoboot=no -V dipsw-serial=yes -V hostcom-driver=stdio -V spc0-id6-image=hd,NetBSD-11.99.5-luna68k-live.img -V spc0-id6-writeignore=1
(PROM) Unknown PROM version
(SCSIHD6) write is ignored
Diagnostic Start.... 11.549'836'096 805b XP(HD647180) WriteInternalIO 53H(DERA) <- $07 (NOT IMPLEMENTED)
11.549'839'519 8060 XP(HD647180) WriteInternalIO 60H(IODRA) <- $50 (NOT IMPLEMENTED)
11.549'842'942 8065 XP(HD647180) WriteInternalIO 70H(DDRA) <- $70 (NOT IMPLEMENTED)
11.650'402'532 8528 XP(HD647180) WriteInternalIO 60H(IODRA) <- $20 (NOT IMPLEMENTED)
11.653'401'080 8bd1 XP(HD647180) ReadInternalIO 48H(T2CSR1) -> $ff (NOT IMPLEMENTED)
11.653'404'340 8bd6 XP(HD647180) WriteInternalIO 48H(T2CSR1) <- $ff (NOT IMPLEMENTED)
11.653'406'785 8bd9 XP(HD647180) ReadInternalIO 49H(T2CSR2) -> $ff (NOT IMPLEMENTED)
11.653'410'045 8bde XP(HD647180) WriteInternalIO 49H(T2CSR2) <- $ff (NOT IMPLEMENTED)
No problems in hardware
OMRON WS Monitor Ver4.25 (Thu May 16 11:54:02 1991) - 0x01000000 bytes of memory
"BSD Monitor-mode"
>k
ctlr: dk
drv: 0
par: c d
fnam: vmunix boot
>g
SPC RESET CONDITION !!
>x

>> NetBSD/luna68k boot, Revision 1.12 (Sat Jan 17 07:19:27 UTC 2026)
>> (based on Stinger ver 0.0 [Phase-31])

Machine model = LUNA-I
Physical Memory = 0x1000000 (16 MB)

sc0 at 0xe1000000: async, parity, ID 7
ID 6: NONO SCSIHD rev 0, 512 bytes/sect x 3145728 sectors
le0: Am7990 LANCE Ethernet, mem at 0x71010000
le0: Ethernet address = 00:00:0a:02:4d:36

Press return to boot now, any other key for boot menu
booting sd(6,0)netbsd - starting in 0 seconds.
type "help" for help.
boot> boot -s
Booting sd(6,0)netbsd (howto 0x2)
2358892+97604 [336560+209989]=0x2dd6c8
[ 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, 2026
[ 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 11.99.5 (GENERIC) #0: Sat Jan 17 16:26:14 JST 2026
[ 1.0000000] tsutsui@proslim:/home/tsutsui/obj.luna68k/sys/arch/luna68k/compile/GENERIC
[ 1.0000000] LUNA-I (MC68030 CPU+MMU, MC68881 FPU)
: