sandbox: Drop distro_boot

This is a complicated set of #defines and it is painful to convert to a
text file. We can (once pending patches are applied) provide the same
functionality with bootmethod. Drop this for sandbox to allow conversion
to a text-file environment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>