use FAT instead of EXT2 in OpenBSD on loongson

[于善伟]

hi:
不好意思，我的英语不行。
I don`t like ext2fs in my *BSD ,I like FAT
install.md:
md_congrats() {
    cat <<__EOT

Once the machine has rebooted use PMON to boot into OpenBSD, as
described in the INSTALL.$ARCH document.
To load the OpenBSD bootloader, use '_boot /dev/fs/ext2@wd0/boot/boot_',
where wd0 is the PMON name of the boot disk.

__EOT
}

'_boot /dev/fat/disk@wd0a/boot/boot_'

I have test it in my Lemote Fuloong 2F mini-PC 
<http://zkml.lemote.com/en/products/mini-computer/2010/0310/111.html>