copy_buildsystem: make sure bitbake directory is copied

copy_buildsystem: make sure bitbake directory is copied

The previous code assumes that bitbake/ directory is under the core layer.
This is the case for Yocto project. But users might clone oe-core and bitbake
separately. So we use bb.__file__ to locate the bitbake directory to make sure
it's copied into the extensible SDK.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>