We see rare failures in eSDK generation with zero sized unihash cache files. This is
almost certainly due to races in the cache file being updated. Add a copy function
where the cache file can be copied with the lock held to avoid this.
(Bitbake rev: 9e72a3915e36cb843037040cb68a82077436dbef)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>