fix(util): Fix detection of the current read-only filesystem.

Switch to stating /usr instead of using ChromeOS's rootdev tool/library.
This should work equally well on both -generic and -usr images.
fix(postinst): Use cgpt from the newly installed image.

The postinst script may depend on the version of cgpt it was built with
so be sure to call it. This is certainly the case in -generic images
which include a cgpt version without support for partition device nodes.
Merge pull request #19 from marineam/usr

fix(util): Fix detection of the current read-only filesystem.