ukify: allow building PE mule

[bluca]

Make the kernel optional too, so that we can easily build and sign a PE mule, that can be used to carry extra command line options.

[bluca]

split out of #27358

[poettering]

"mule" sounds like it would be something that would transport cocaine over the border...

can't we find a less crime-affine name?

[poettering]

hmm, the primary job of ukify should still be building UKIs, hence I would make this functionality a bit harder to reach when just forgetting to specify thing.

i.e. add a new switch to build such images. or make it so that --linux=/dev/null selects this mode.

[bluca]

"mule" sounds like it would be something that would transport cocaine over the border...

can't we find a less crime-affine name?

I think they use submarines these days

[bluca]

hmm, the primary job of ukify should still be building UKIs, hence I would make this functionality a bit harder to reach when just forgetting to specify thing.

i.e. add a new switch to build such images. or make it so that --linux=/dev/null selects this mode.

There's no --linux option, it's positional. Forcing to pass a random /dev/null looks quite bad from usability POV.

[poettering]

There's no --linux option, it's positional. Forcing to pass a random /dev/null looks quite bad from usability POV.

Ah, right. But then at least add --no-linux or so

[poettering]

code-wise looks all good to me. will leave thorough review to @keszybz

[keszybz]

are specified → is specified

[keszybz]

Let's merge this. I'll need to rebase #27262 to adjust to this, I can fix the one grammar nit then too.

[keszybz]

FWIW, I like "mule".