fix(coreos-base/bootengine): change to build single image

don't build an image for every boot kernel, just build a single
uncompressed image and put it in /usr/share/bootengine/bootengine.cpio.
Merge pull request #78 from philips/cleanup-bootengine

fix(coreos-base/bootengine): change to build single image
fix(coreos-base/bootengine): don't hardcode ${ROOT}

use the ${ROOT} variable instead of hard coding our own right here.
Merge pull request #79 from philips/remove-root-hack

fix(coreos-base/bootengine): don't hardcode ${ROOT}