CHROMIUM: init: don't dm_substitute_devices().

CHROMIUM: init: don't dm_substitute_devices().

With uuid+offset support in dm_get_device(), we don't need to substitute at all
here, and this code was kind of a wart.

BUG=chromium-os:17184
TEST=Adhoc
Booted. dmesg | grep device-mapper
Built a normal image for kaen, booted it successfully. This tests u-boot.
Build a normal image for any x86 board (I used x86-alex). Boot it. This tests
chromeos-firmware boot.
Modify your x86 image for vm using image_to_vm.sh (this doesn't work for Arm
yet). Make sure it boots in the VM. This tests legacy (syslinux) boot.
As for testing grub boot, you're on your own :)

Change-Id: I6c7f048a7ec8b6949a29c86aaa5b9ebec6545404
Reviewed-on: http://gerrit.chromium.org/gerrit/3970
Signed-off-by: Elly Jones <ellyjones@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/6063
Reviewed-by: Will Drewry <wad@chromium.org>