buildhistory: change default BUILDHISTORY_DIR to TOPDIR

* like PERSISTENT_DIR I find BUILDHISTORY_DIR more persistent
  then tmp-eglibc directory
* WARN: people with existing checkouts in TMPDIR/buildhistory
  need to move it to new location first (in order to save history)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>