Previously we use the file time stamp to judge if a cache is valid.
Here this commit introduce a new method, which calculates the total
hash value for a certain configuration's key/value paris, and tag
it into cache filename, for example, bb_cache.dat.xxxyyyzzz.
This mechanism also ensures the cache's correctness if user
dynamically setting variables from some frontend GUI, like HOB.
(Bitbake rev: 1c1df03a6c4717bfd5faab144c4f8bbfcbae0b57)
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>