Use consistent mtime timestamps

This patch adds consistent_timestamps.bbclass which is
a hackish way of resembling reproducible build -
byte-for-byte identical binary packages from a given source.
Naturally this is not a complete solution, but it fixes
the issue described below. Real solution for this should
be done by the Yocto project itself.

In OSTree each new update is checked out with all files and
directories having mtime=0, this breaks fontconfig cache
validity check (mtime embedded in the cache should match
the containing directories mtime). Yocto generates this cache
file in poky/meta/lib/oe/rootfs.py::create by calling
self._run_intercepts() after all packages have been installed.
These timestamps are nondeterministic as they depend on current
system time.

By using ROOTFS_POSTINSTALL_COMMAND hook we can ensure that
these embedded time stamps have a known time, which is a basic
property of reproducible build system.

Change-Id: Ib2f130248f2e65db391d2b2f19ab5dac30a2cfb0
Task-number: QTEE-1081
Reviewed-by: Samuli Piippo <samuli.piippo@theqtcompany.com>

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