EFI stub: add LoaderImageIdentifier if non-existant

EFI stub: add LoaderImageIdentifier if non-existant

when an image with the stub is booted directly from UEFI, bootctl status
shows this:
```
Current Loader:
      Product: n/a
          ESP: /dev/disk/by-partuuid/b0a0807d-0592-40e9-adac-3bb724e9e305
         File: └─n/a
```

After this change, the stub reports the image itself:
```
Current Loader:
      Product: n/a
          ESP: /dev/disk/by-partuuid/b0a0807d-0592-40e9-adac-3bb724e9e305
         File: └─/EFI/Secure/secure-boot-4.14.8+.efi
```