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>

arch/sandbox/include/asm/state.h

@@ -93,6 +93,7 @@ struct sandbox_state {
 	bool ram_buf_read;		/* true if we read the RAM buffer */
 	bool run_unittests;		/* Run unit tests */
 	const char *select_unittests;	/* Unit test to run */
+	bool handle_signals;		/* Handle signals within sandbox */
 
 	/* Pointer to information for each SPI bus/cs */
 	struct sandbox_spi_info spi[CONFIG_SANDBOX_SPI_MAX_BUS]