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

[LEDE Bugs]

The following task has a new comment added:

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

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Rebased & merged to my staging tree at http://git.lede-project.org/e1c3bb6
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=264#comment886