sandbox: Don't try distro_bootcmd by default

For the distro_bootcmds to succeed on the sandbox a bit of setup is
required (e.g. network configured or host image bound), so running them
by default isn't that useful.

Add a -b/--boot command to the sandbox binary, which triggers the
distro_bootcmds to run after the other command-line commands.

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