add(coreos-*): Add postinst and setgoodroot scripts.

These replace code in the postinst and setgoodroot code in
platform/installer. The setgoodroot was already a shell script but this
new version uses cgpt's new ability to handle partition device nodes
directly. The postinst script replaces a large pile of legacy C++ code
from ChromeOS and should work with both generic and usr images.
Merge pull request #18 from marineam/postinst

add(coreos-*): Add postinst and setgoodroot scripts.