branches: 1.1.2; 1.1.4;
wii: Add standalone bootloader.

This adds "ppcboot.elf" to the Wii (evbppc) build, which can be placed on
the FAT partition of an SD card in the bootmii directory. The bootloader
doesn't have a user interface, it simply loads a kernel from the first
FFS partition it finds on the SD card.

Debug logs are printed to a USB Gecko device in either memory card slot
if present.

\\-__,------,___.
\\ __,---` NetBSD/evbppc Wii MINI boot
\\ `---,_. Revision 1.0 (Sun Nov 16 19:27:47 UTC 2025)
\\-,_____,.---`
\\
\\
\\

booting netbsd 7828064/-|+130520 [315696+305916]=0x82f350
[ 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 11.99.3 (WII) #207: Sun Nov 16 14:28:23 AST 2025
[ 1.0000000] jmcneill@berserk:/home/jmcneill/netbsd/obj/sys/arch/evbppc/compile/WII
[ 1.0000000] total memory = 90112 KB
[ 1.0000000] avail memory = 77008 KB
[ 1.0000000] mainbus0 (root): Nintendo Wii
...