[efi] Unload started images only on failure

If the StartImage() call returns with no error, then the image must
have been started and returned successfully.  It either unloaded
itself, or it intended to remain loaded (e.g. it was a driver).  We
therefore do not unload successful images.

If there was an error, we attempt to unload the image.  This may not
work.  In particular, there is no way to tell whether an error
returned from StartImage() was due to being unable to start the image
(in which case we probably should call UnloadImage()), or due to the
image itself returning an error (in which case we probably should not
call UnloadImage()).  We therefore ignore any failures from the
UnloadImage() call itself.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Fill in loaded image's DeviceHandle if firmware fails to do so

Some EFI 1.10 implementations (observed with a mid-2011 iMac) seem to
fail to fill in the DeviceHandle for our loaded images.  It is
plausible that these implementations fill in the DeviceHandle only if
loading the image from a device path (rather than directly from a
memory buffer).

Work around this problem by filling in DeviceHandle if the firmware
leaves it empty.

We cannot sensibly fill in FilePath, because we have no way of knowing
whether or not the firmware will treat this as a pointer to be freed
when the image returns.

Reported-by: Curtis Larsen <larsen@dixie.edu>
Tested-by: Curtis Larsen <larsen@dixie.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>