buildhistory.bbclass: create the buildhistory directory when needed

When the BUILDHISTORY_RESET is enabled we need to move the
content from BUILDHISTORY_DIR to BUILDHISTORY_OLD_DIR but
when we start a clean build in the first run we don't have the
BUILDHISTORY_DIR so the move of files will fail.

| ERROR: Command execution failed: Traceback (most recent call last):
|  File "/xxx/poky/bitbake/lib/bb/command.py", line 110, in runAsyncCommand
|    commandmethod(self.cmds_async, self, options)
|  File "/xxx/poky/bitbake/lib/bb/command.py", line 564, in buildTargets
|    command.cooker.buildTargets(pkgs_to_build, task)
|  File "/xxx/poky/bitbake/lib/bb/cooker.py", line 1481, in buildTargets
|    bb.event.fire(bb.event.BuildStarted(buildname, ntargets), self.databuilder.mcdata[mc])
|  File "/xxx/home/builder/src/base/poky/bitbake/lib/bb/event.py", line 214, in fire
|    fire_class_handlers(event, d)
|  File "/xxx/poky/bitbake/lib/bb/event.py", line 121, in fire_class_handlers
|    execute_handler(name, handler, event, d)
|  File "/xxx/poky/bitbake/lib/bb/event.py", line 93, in execute_handler
|    ret = handler(event)
|  File "/xxx/poky/meta/classes/buildhistory.bbclass", line 919, in buildhistory_eventhandler
|    entries = [ x for x in os.listdir(rootdir) if not x.startswith('.') ]
| FileNotFoundError: [Errno 2] No such file or directory: '/xxx/buildhistory'

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>