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For the two different platforms we support in the Allwinner port we
mostly rely on header files covering the differences. This leads to the
platform.mk files in the respective directories to be almost identical.

To avoid further divergence and make sure that one platform doesn't
break accidentally, let's create a shared allwinner-common.mk file and
include that from the platform directory.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>