If the mirrors code is trying to create a symlink and the
parent directory doesn't exist, as might be the case for sstate
mirrors where the fetch is into a subdir, it can silently fail.
Ensure the directory exists in this case to avoid issues.
(Bitbake rev: eff16e474ee7dc49ae433420a4c8d15d3314a618)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>