config_distro_bootcmd.h: Add shared block definition for the host interface

Define the common shared block environment for the host interface in
preperation for the sandbox build to use config_distro_bootcmd.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>