external: Refactor Makefile so it is easier to just build one arch

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: utils: Skip building openssl CLI

This causes intermittent link failures during parallel builds, but since
we don't need it anyway, it's easier just to turn it off.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
travis: Limit native builds to ARM only

The full build exceeds travis-ci's 4MB log length limit and takes
>>25min.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>