rootdev: Strip partitions less aggressively In device names like dm-N or loopN, the N is not a partition number and should not be stripped off. These can arise for example if run inside a chroot mounted on a loopback device. BUG=chromium:748665, chromium:730144 TEST=New unit tests. Change-Id: If7ecacbfcf00690376cb4ffc75baa45422579085 Reviewed-on: https://chromium-review.googlesource.com/585652 Commit-Ready: Benjamin Gordon <[email protected]> Tested-by: Benjamin Gordon <[email protected]> Reviewed-by: Simon Glass <[email protected]> Reviewed-by: Mike Frysinger <[email protected]>