sandbox: Disable CONFIG_DISTRO_DEFAULTS

This is not used for sandbox, so drop it. Enable the things that it
controls to avoid dstrastic changes in the config settings for
sandbox builds.

The end result is that these are enabled:

   BOOTMETH_DISTRO
   BOOTSTD_DEFAULTS

and these are disabled:

   USE_BOOTCOMMAND
   BOOTCOMMAND (was "run distro_bootcmd")
   DISTRO_DEFAULTS

Note that the tools-only build has already disabled DISTRO_DEFAULTS
and BOOTSTD_FULL

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
---
Changes in v4:
- Only modify sandbox and restrict the changes to only DISTRO_DEFAULTS