sandbox: add config_distro_defaults and config_distro_bootcmd

sandbox: add config_distro_defaults and config_distro_bootcmd

Make the sandbox setup more generic/examplary by including
config_distro_defaults.h and config_distro_bootcmd.h.

Among other things this makes it easy to test whether images will boot
though with the standard distro bootcmds by running e.g:
  u-boot -c 'host bind 0 myimage.img ; boot'

By default there are 2 target host devices to emulate device with
multiple storage devices (e.g. internal ("host 0") and external
("host 1") and verify that the prioritization and fallbacks do work
correctly.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>