populate_sdk_ext: Fix race condition on bb_unihashes.dat

populate_sdk_ext: Fix race condition on bb_unihashes.dat

There is a race were the bb_unihashes.dat file may end up zero sized due to
concurrent builds. Use recently added API within bitbake to copy the file
safely. Also use the opportunity to remove hardcoded filepaths internal to
bitbake from OE-Core.

Bump the minimum bitbake version to match the API being used.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>