buildhistory.bbclass: remove out-dated information on request

buildhistory.bbclass by design is incremental: each build adds or
updates information. Information is never removed.

Sometimes it can be useful to reduce the information only to those
recipes that were build during a specific bitbake invocation, for
example when the invocation does a full world build.

This is now possible by setting BUILDHISTORY_RESET as explained in the
modified class. The comment on the variable also mentions the caveats
associated with using this mode.

In this mode, buildhistory.bbclass first moves all existing
information into a temporary directory called "old" inside the build
history directory. There the information is used for the "version
going backwards" QA check. Then when the build is complete and before
(potentially) committing to git, the temporary directory gets deleted.

Because information that has not changed during the build will be
reconstructed during full world rebuilds, a git log will then only
show real updates, additions and removals.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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