efi_loader: fix AppendDevicePath

The logic of the AppendDevicePath service of the
EFI_DEVICE_PATH_UTILITIES_PROTOCOL is incorrectly implemented:

* if both paths are NULL an end node has to be returned
* if both paths are not NULL the end node of the second device path has to
  be kept

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>