Fix EFI multi device boot support

Fix EFI boot support when presented with multiple valid boot partitions
across multiple devices.

It now prefers to boot from partitions that are present on the underlying
device that the boot1 image was loaded from. This means that it will boot
from the partitions on device the user chose from EFI boot menu in
preference to those on other devices.

Also fixed is the recovery from a failed attempt to boot, from a seemingly
valid partition, by continuing to trying all other available partitions
no matter what the error.

boot1 now use * to signify a partition what was accepted from the preferred
device and + otherwise.

Finally some error messages where improved and DPRINTF's with slowed boot
to aid debugging.

ZFS will still be preferred over UFS when both are available on the boot
device.

Reviewed by: imp
MFC after: 1 week
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D5108