as this only allows us to access file systems that EFI knows about.
With a loader that can only use EFI-supported file systems, we're
forced to put /boot on the EFI system partition. This is suboptimal
in the following ways:
1. With /boot a symlink to /efi/boot, mergemaster complains about
the mismatch and there's no quick solution.
2. The EFI loader can only boot a single version of FreeBSD. There's
no way to install multiple versions of FreeBSD and select one
at the loader prompt.
3. ZFS maintains /boot/zfs/zpool.cache and with /boot a symlink we
end up with the file on a MSDOS file system. ZFS does not have
proper handling of file systems that are under Giant.
Implement a disk device based on the block I/O protocol instead and
pull in file system code from libstand. The disk devices are really
the partitions that EFI knows about.
This change is backward compatible.
MFC after: 1 week
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