loader: lua: test that /boot exists first

[manu]

Summary

Otherwise on fs like tftp where no directory listing is possible we fail
on the .dir method.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG

[kevans]

stand/lua/core.lua

251

Any objection to moving this up above the autodetect bailout and doing something like...

if not lfs.attributes("/boot", "mode") then
    autodetect = "no"
    loader.setenv("kernels_autodetect", "NO")
end

?

Two benefits:

1. adds a way to detect that autodetection was skipped rather than it failed
2. less important, avoid a second stat() on /boot if we re-enter this (e.g., drop to loader prompt and re-enter)

[imp]

I'd prefer that we had a variable for "/boot" but that's orthogonal to this change.
However, we need a comment I think for why we do this.
I approved, but that's contingent on expanding the comment to explain why we short-circuit.

stand/lua/core.lua

251

I'm surprised we don't have a variable for "/boot"
However, you should add a comment above explaining why we short-circuit here, drawn from the commit message. It's weird enough that I think explaining why here will likely avert churn in this area in the future.

[imp]

stand/lua/core.lua

251

hmmm, I like kyle's notion better.

[manu]

stand/lua/core.lua

251

Sure, will change this.

[manu]

Move check up to disable autodetect.

[kevans]

Thanks!