Backport CI support to bullseye
Cherry-pick most of the changes that introduced CI support between bullseye and bookworm. The notable differences from bookworm are:
- This doesn't fix build flags for user-space tools, and does not enable the blhc job.
- Another lintian error needs to be overridden because what we're doing is correct for bullseye.
- There are different kconfig errors to fix.