Self-invalidate bootstrap cache upon mock config change

[praiskup]

Mock should mock --scrub=bootstrap (plus bootstrap root cache) when some of the
config files affecting target chroot (the -r option) changes.

Originally requested here: https://bugzilla.redhat.com/show_bug.cgi?id=2258570

[praiskup]

Re-reported here: https://bugzilla.redhat.com/show_bug.cgi?id=2263260

[praiskup]

Re-reported: https://bugzilla.redhat.com/show_bug.cgi?id=2264405

[praiskup]

Brain dump:

• the commands.clean() is called before the build happens (rebuild_generic)
• the commands.clean() is called before --init (if not configured otherwise)
• the commands.clean() doesn't call bootstrap.delete() yet, see The --scrub= and --clean affects bootstrap, too #1454
• root cache is created for bootstrap chroot
• bootstrap image is downloaded and extracted only if not yet initialized
• bootstrap image is not downloaded and extracted in case of root cache hit (extracting root cache makes the buildroot into the "was_initialized" state
• bootstrap_image_ready means that many options have no effect on bootstrap at all

All that said, the cache invalidation already exists. But! Because commands.clean() doesn't affect bootstrap (yet?), it has no effect. And, even if we invalidate the caches -> bootstrap image extraction would probably override most of the things (unless --no-bootstrap-image is specified).

[praiskup]

Ohh, and:

• The --scrub=root-cache is not cleaning the bootstrap root cache.
• The --scrub=yum-cache is not scrubbing bootstrap yum, etc.