[FS#840] AR71xx - Recent firmware won't flash (building without errors)

[LEDE Bugs]

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FS#840 - AR71xx - Recent firmware won't flash (building without errors)
User who did this - juanriccio (juanriccio)

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Mathias Kresin and Piotr Dymacz, thanks a lot!

It took me some time to get around to it, but when I tried to follow your 
advice, the problem was solved.

As you suggested, it must have been the previous image that was missing some 
crucial component for sysupgrade.

Here's my report of how I managed to fix the problem.

I followed the instructions here:
https://lede-project.org/docs/user-guide/failsafe_and_factory_reset
to get the router booted in failsafe on 192.168.1.1.
Then I SSH-ed into the router using PuTTY, downloaded a freshly compiled image 
into /tmp, and flashed it using mtd.

I verified that the new image can indeed be sysupgraded via LuCi.

Note - My previous unupgradable image was self-built, indeed, but I only made a 
few changes from default, and I took away only stuff like PPP/PPPoE. The 
present, working image, is also self-built, from the very same config.seed.

Thanks again for your help. Without you, I'd never figured out the mtd part.

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=840#comment4173