dm: sandbox: Switch over to using the new host uclass

dm: sandbox: Switch over to using the new host uclass

Update the sandbox implementation to use UCLASS_HOST and adjust all
the pieces to continue to work:

- Update the 'host' command to use the new API
- Replace various uses of UCLASS_ROOT with UCLASS_HOST
- Disable test_eficonfig since it doesn't work (this should have a unit
  test to allow this to be debugged)
- Update the blk test to use the new API
- Drop the old header file

Unfortunately it does not seem to be possible to split this change up
further.

Signed-off-by: Simon Glass <sjg@chromium.org>