[dpdk-dev] mk: make config enhancements

[David Hunt]

This patch series is a couple of small patches to make the 'make config'
of the build a bit easier for users.

Users can now 'make defconfig' which will pick a sensible default based on
some 'uname' queries of the system.

Users can also set RTE_TARGET on  in their environment which will get picked
users type 'make config' without T=template.

There's still some work to be done on non-x86 machines so I'd appreciate
some suggestions of tweaks to make this patch function on those as well.