Chromium OS version of rootdev

Inspired by git://git.debian.org/~lamont/util-linux.git / 717db2c8177203fe242ea35b31bc312abe9aa3c9

Description: Performs operations to discover and annotate the root block device

Prior to this commit, rootdev was derived from the above link.  However, it has
been rewritten in its entirety and licensed as per LICENSE.

chromeos-rootdev:
- Provides core functionality in a library: librootdev
- Walks sysfs to discover the block devices
- Supports resolving through to /sys/block/XXX/slaves/*/dev devices
- Will test and, optionally, symlink to the /dev entry for standard devices.
- Is testable.