build: establish an invariant machine type

build: establish an invariant machine type

Add the machine definition 'default' which is special compared
to 'native' (most optimized for current system) or any explicit
type (external entity has to decide on the type).

It defaults to the per arch agreed common minimal baseline
needed for DPDK to reasonable work.

That might not be the most optimized, but the most portable
version while still being able to support the CPU features
required for DPDK.

Going forward this can be bumped up by the DPDK project, but it
can never be an invariant like 'native'.

Distributions and other needing portable code are expected to
define the machine as 'default'.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>