[FS#380] sysupgrade corrupts firmware on ASUS RT-N16

[LEDE Bugs]

The following task has a new comment added:

FS#380 - sysupgrade corrupts firmware on ASUS RT-N16
User who did this - Nikolay Amiantov (abbradar)

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I'll be able to provide a serial log after I get USBUART (should be in few days). 

Regarding dumping partitions -- can sysupgrade be asked to start some kind of failsafe mode (presumably from tmpfs) after it finishes instead of rebooting? To dump them after TFTP I'd need to boot into LEDE first (I don't know of any method to boot initramfs from somewhere on this device; is there one?) To save pristine state of the partition would it suffice to boot to failsafe mode (so that nothing is mounted rw) and dump from there? 
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=380#comment1231