stage1: componentize stage1 rootfs construction and use make

 * Move stage1/*.go to stage1/init/

 * Split stage1/mkrootfs.sh's logical components into subdirs of
   stage1/rootfs:

   - Deriving tree from CoreOS image and caching: rootfs/usr
     * Cache management in cache.sh
     * File extraction and picking in mkbase.sh
     * File inclusions split under usr/manifest.d

   - Fakesdboot: rootfs/shim/
     * Renamed to 'shim' since we've gone past just tricking nspawn

   - Root compositing and aggregation of odds and ends: rootfs/aggregate

 * Introduction of Makefiles for every component and a simple recursive
   parallel-capable make for the rootfs tip.

 * Update toplevel build script to invoke make on stage1/rootfs and find
   init sources in stage1/init, .gitignore additions as well.

 * Removed stage1/mkrootfs.sh

 Since the image cache has moved there will be an image download on first build
 in the new arrangement.  The caching has also been improved to handle CoreOS
 image revision changes by naming the cached signature with a hash of the URL.
stage1/rootfs/shim: addition of parens to silence new use of gcc -Wall