lualoader: fix pruning of non-existent default kernel

Removing the kernel from the list of available kernels is sufficient to
avoid rendering it in the list, but we need more for booting to actually

lualoader: fix pruning of non-existent default kernel

Removing the kernel from the list of available kernels is sufficient to
avoid rendering it in the list, but we need more for booting to actually
work. Notably, the 'kernel' loader.conf var was left intact to its
default value, so if one didn't use the kernel selector in the menu then
we'd try to boot the nonexistent 'kernel' instead of the new default
(first autodetected).

There's room to improve the error messages here, but for now let's just
make it actually work correctly.

PR: 292232
Fixes: d04415c520b03 ("loader: lua: remove the default kernel [...]")

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