init: add root=PARTUUID=UUID/PARTNROFF=%d support

Expand root=PARTUUID=UUID syntax to support selecting a root partition by
integer offset from a known, unique partition.  This approach provides
similar properties to specifying a device and partition number, but using
the UUID as the unique path prior to evaluating the offset.

For example,
  root=PARTUUID=99DE9194-FC15-4223-9192-FC243948F88B/PARTNROFF=1
selects the partition with UUID 99DE.. then select the next
partition.

This change is motivated by a particular usecase in Chromium OS where the
bootloader can easily determine what partition it is on (by UUID) but
doesn't perform general partition table walking.

That said, support for this model provides a direct mechanism for the user
to modify the root partition to boot without specifically needing to
extract each UUID or update the bootloader explicitly when the root
partition UUID is changed (if it is recreated to be larger, for instance).
 Pinning to a /boot-style partition UUID allows the arbitra...