src.conf: add WITH_TOOLCHAIN description

[emaste]

Summary

It is not used by the in-tree default configuration, but adding it
allows downstream projects with different defaults to make use of
Cirrus-CI.

(Posting this one for review for the general idea; if this is generally agreeable I'll just add others that I come across.)

[imp]

Looks good to me.
IMHO, we shouldn't have WITH and WITHOUT files, but just TOOLCHAIN, etc and reword the options if needed to cope with any asymmetry between WITH and WITHOUT phrasing today that's relevant.
But this change is great,

[emaste]

IMHO, we shouldn't have WITH and WITHOUT files, but just TOOLCHAIN, etc and reword the options if needed to cope with any asymmetry between WITH and WITHOUT phrasing today that's relevant.

I agree we should do something like this (after Kyle's rework), as long as we can avoid reducing clarity. At the very least we should be able to remove one of the pair when we have both WITH_ and WITHOUT_ descriptions.

[brooks]

tools/build/options/WITH_TOOLCHAIN

2