[fallback] For HD() device paths, use just the media node and later.

UEFI 2.x section 3.1.2 provides for "short-form device path", where the
first element specified is a "hard drive media device path", so that you
can move a disk around on different buses without invalidating your
device path.  Fallback has not been using this option, though in most
cases efibootmgr has.

Note that we still keep the full device path, because LoadImage()
isn't necessarily the layer where HD() works - one some systems BDS is
responsible for resolving the full path and passes that to LoadImage()
instead.  So we have to do LoadImage() with the full path.
[fallback] Attempt to re-use existing entries when possible.

Some firmwares seem to ignore our boot entries and put their fallback
entries back on top.  Right now that results in a lot of boot entries
for our stuff, a la https://bugzilla.redhat.com/show_bug.cgi?id=995834 .

Instead of that happening, if we simply find existing entries that match
the entry we would create and move them to the top of the boot order,
the machine will continue to operate in failure mode (which we can't
avoid), but at least we won't create thousands of extra entries.

Signed-off-by: Peter Jones <pjones@redhat.com>