path.py: Deal with race issue

path.py: Deal with race issue

The change to use copyhardlinktree in some of the sstate code instead of
copytree exposed a race condition. This is due to cp failing if it finds
a directory doesn't exist yet some other process creates it while cp was
trying to create it itself. tar doesn't error in this case.

To fix this we need to create the directory structure with tar, then
use cp to hardlink the files. Messy but probably worth doing.

I also took the opportunity to remove src_bak since the code is neater
without it.

(From OE-Core rev: 2f954a9a6932f1e6c564e7e7aacaac628a75eed7)

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