[FS#264] xen_domu images need change to grub.cfg to boot unattended

[LEDE Bugs]

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FS#264 - xen_domu images need change to grub.cfg to boot unattended
User who did this: Jo-Philipp Wich (jow-)
Status: Unconfirmed -> Requires testing

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=264