feat(build_image): install the au-key

Install the developer au key on production images. This will need to
change when we have the production key.
fix(cros_make_image_bootable): no more bootkernel

the bootkernel is going away and we will just have a single kernel for
all of the things. This is because of the kxec on xen problem.
Merge pull request #45 from philips/switchup-bootkernel

Switchup bootkernel