* Avoid trying to boot non-existing kernel image in fail-over case
The A/B update system automatically switches to the other boot slot when
booting fails. However, in a fresh installation, only boot slot A
exists. If booting fails three times (e.g. if somebody plugs out power
before the slot can be marked as good), then the system switches to boot
slot B which does not contain a kernel image yet. Avoid trying to boot
the non-existing kernel image.
With this change, if slot B is empty U-Boot will restore both slots to 3
attempts and retry booting from slot A on next reboot:
```
Trying to boot slot B, 2 attempts remaining. Loading kernel ...
** Unrecognized filesystem type **
No valid slot found, resetting tries to 3
storing env...
```
Co-authored-by: Pascal Vizeli <pascal.vizeli@syshack.ch>
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