sandbox: Support signal handling only when requested

At present if sandbox crashes it prints a message and tries to exit. But
with the recently introduced signal handler, it often seems to get stuck
in a loop until the stack overflows:

Segmentation violation

Segmentation violation

Segmentation violation

Segmentation violation

Segmentation violation

Segmentation violation

Segmentation violation
...

The signal handler is only useful for a few tests, as I understand it.
Make it optional.

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