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Updating to the 1.15 release.

With this update there are significant new build constraints and
dependencies. As such, the cross binaries are now being be built
by default, with the old (non-cross) build being kept around as
an optional way to build the components.

There are still issues with the non-cross build, so it will
require more work in the future.

We also document the requirement for selinux and seccomp in the
README as dependencies if you build cri-o.
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>