[FS#685] x86: sysupgrade to LEDE lose the configuration

[LEDE Bugs]

The following task has a new comment added:

FS#685 - x86: sysupgrade to LEDE lose the configuration
User who did this - Matthias Schiffer (NeoRaider)

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I have some working code in my staging tree at https://git.lede-project.org/?p=lede/neoraider/staging.git;a=shortlog;h=refs/heads/sysupgrade now that changes sysupgrade to use the staged sysupgrade mechanism on all targets. I have verified that it works correctly on ar71xx, x86 and ramips (nand); it will need more testing, but it should work on the other targets, too.

On top of the sysupgrade changes, which don't touch target-specific code at all, the branch also contains the following:
  * The call to the NAND sysupgrade code is simplified on ramips (a similar change should be done in all NAND targets eventually, so we can get rid of a few hacks in sysupgrade)
  * partx calls are added to the x86 sysupgrade to make the partition table updates more robust. This should be done on all targets using harddisk-like storage with partition tables.

TODO:
  * Re-add some missing features to sysupgrade (e.g. support for image download via HTTP)
  * Add more sanity checks (ensure that the image is in /tmp, that the rootfs has been unmounted, ...)

I will also backport my work to OpenWrt CC and verify that it really fixes this issue, as we need working sysupgrade from CC to LEDE in Gluon.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=685#comment2468