[efi] Unload started images only on failure

[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>