centos.aarch64 creates a CentOS 8 image.
Also added a new kickstart script used to build the centos.aarch64 image.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Peter Puhov <peter.puhov@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200601211421.1277-7-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-11-alex.bennee@linaro.org>