Add _kill() signal handler to cause a fault, so assert() lights up the red led on failure.
Add _kill() signal handler to cause a fault, so assert() lights up the
red led on failure.

firebitsbr/bare-metal-arm,mbains/bare-metal-arm,payne92/bare-metal-arm,payne92/bare-metal-arm,mbains/bare-metal-arm,payne92/bare-metal-arm,firebitsbr/bare-metal-arm