buildhistory: Allow customising buildhistory tags

Allow setting custom buildhistory tag prefixes. This allows multiple
build directories to share one buildhistory git repository with multiple
worktrees.

(From OE-Core rev: 1bb2aa53a74e36dc3ba901b9d8ce780e7880cef8)

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>