Revert to FAT12. This file system is apparently too small for FAT32, even
if the old (pre r264889) newfs_msdos allowed it. And FAT12 seems to work
perfectly well.
Provide a proper armv7 implementation of icache_sync_all rather than
using armv7_idcache_wbinv_all, because wbinv_all doesn't broadcast the
operation to other cores.  In elf_cpu_load_file() use icache_sync_all()
and explain why it's needed (and why other sync operations aren't).

As part of doing this, all callers of cpu_icache_sync_all() were
inspected to ensure they weren't relying on the old side effect of
doing a wbinv_all along with the icache work.
Add script to setup bootable CD ISOs for both BIOS and EFI systems. Tested
and working on QEMU. Actually using this script as the regular image
generator, like with the memstick one, will require that the kernel support
EFI too. In particular, the following two things are required:
1. vt(9) be the default console driver
2. vt_efifb and vt_vga be able to coexist usefully in the same kernel

One other note here is that this requires newfs_msdos and mdconfig, which is
really ugly. NetBSD's makefs at least seems to support FAT now. If that
actually works, it should be imported and we can get rid of the mdconfig mess.