imagefeatures: Add test to blacklist building busybox

imagefeatures: Add test to blacklist building busybox

Now that we have a packagegroup that can be used to replace the overall
functionality of busybox as base-utils, add a test that we can continue
to build a fairly complex image without busybox being available.  This
also doubles as documentation-by-test of how to do this.

Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>