rootdev: fix -d, add -c and -r. ifdefs for so use

Multiple changes:
- Ensure the -d flag trims off all of the partition info and only if there is a partition
- Add -c, device node creation for platform-agnostic /dev/ROOT, /dev/ROOT0 and /dev/ROOT1
- Add -r, resolve the first slave if the device is a device-mapper device
- Add -s, create a symlink from the found device to /dev/ACTIVE_ROOT
- Adds short-circuiting if /dev/ACTIVE_ROOT exists to both dm resolution and
  normal lookups
- Add support for building a library with a reusable header and interfaces

I'll follow this up with a change to install <rootdev/rootdev.h> and the .so in
the ebuild if the interface is at all interesting.  If prefered, I can just add:
rootdev -s -r to chromeos_startup, then AU can just rely on readlink(/dev/ACTIVE_ROOT).

TEST=built for x86-generic testing with vroot: -r, -s, -c, -d and combos
     switching to non-vroot to test now
     can someone test arm for me?

Change-Id: Ibab8072afb012ea77d457517f1849e0917d02892

Review URL:
9 files changed