Support source statement in loopback.cfg

Now `loopback.cfg` is using `source` in `nitrux_release_testing.iso` 2018100420552200:

```
me@host:~$ cat /boot/grub/loopback.cfg
source /boot/grub/grub.cfg
```

This is allowed as per https://www.supergrubdisk.org/wiki/Loopback.cfg, but I need to make SystemImageKit recognize it.