cmake.bbclass: remove ${B} before cmake_do_configure

It is fallible to remove ${B} in directory ${B} itself. And it does fail
when call bitbake by third-party wrapper script.

Use flag 'cleandirs' to remove ${B} first if build out of source tree.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>