sstate: Use -m option to tar when unpacking sstate
We've noticed failures on the project autobuilders where a shared sstate
directory is used across multiple builders and the clocks become skewed.
Most of the time this causes harmless building but if this happens where
an environment is changed (make install vs make in qt4-x11-free for example),
the build can fail.
This avoids modification times in the future and should make builds safer
in shared environments sstate was designed for.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
