sandbox: Quit when hang() is called

sandbox: Quit when hang() is called

It doesn't make a lot of sense to hang on sandbox when hang() is called,
since the only way out is Ctrl-C. In fact, Ctrl-C does not work if the
terminal is in raw mode, which it will be if the command-line has not been
reached yet. In that case, Ctrl-Z / kill -9 must be used, which is not
very friendly.

Avoid all of this by quiting when hang() is called.

Signed-off-by: Simon Glass <sjg@chromium.org>