loader.efi: Expand uefi_rootdev to probe disks

uefi_rootdev had to be the exact device to work. This is often difficult
to arrange and awkward. Instead, have it search the specified device: As
a partition if it specifies the partition, and as the whole disk if the
specified partition didn't work or it specifies a disk. This brings it
into lone what we do with other devices. This overrides the boot manager
search we do before we do this for the boot device, but this knob has
always been meant to overload the boot manager search.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D57803