U-boot script does not cope with missing kernel, should trigger recovery

[zyga]

I've discovered that my Ubuntu Core 20 system, running on a Raspberry Pi 3B, has failed to boot for several weeks. After a long debugging session I've noticed that:

1. The system expected some version of the kernel snap to be installed
2. That version was not on disk (ext4) and not extracted to boot partition (fat)
3. The boot script hanged after failing to load the kernel.

I had no traces of information as to what has happened (no persistent journal, all changes and tasks recycled). A complete thread of debugging/analysis is shared on twitter: https://twitter.com/zygoon/status/1548640642967928833