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

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Wilfried Klaebe (Wonka) 

Attached to Project - LEDE Project
Summary - xen_domu images need change to grub.cfg to boot unattended
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Hello.

I'm using Gluon 2016.2 (GLUON_TARGET=x86-xen_domu), which currently still uses OpenWrt, but as many of us know, Gluon will switch to LEDE.

After both original installation and sysupgrading, it does not boot but hangs in the pvgrub2 bootloader, unable to find vmlinuz. This is because in the Xen domU, pvgrub doesn't know hd0, but only xen/xvda. The line »set root='(hd0,msdos1)'« in /boot/grub/grub.cfg in the first partition in the image needs to be »set root='(xen/xvda,msdos1)'« in xen_domu images.

Relevant parts from xen .cfg file for this domU:
---88---

Relevant file in LEDE repo:
https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/x86/image/grub.cfg


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