Adepted to the new bitbake cache mechanism which is based on file
content but not on file timp stamps any more.
(From OE-Core rev: c1705317f9456f761da2094e886a07939291e53a)
Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>