fw: Protect zeroisation against compiler optimisation.

The memset() responsible for the zeroisation of the secure_ctx under
the compute_cdi() function in FW's main.c, was optimised away by the
compiler. Instead of using memset(), secure_wipe() is introduced
which uses a volatile keyword to prevent the compiler to try to
optimise it. Secure_wipe() is now used on all locations handling
removal of sensitive data.

hw/application_fpga/fw/tk1/lib.c

@@ -155,3 +155,10 @@ int memeq(void *dest, const void *src, size_t n)
 
 	return res;
 }
+
+void secure_wipe(void *v, size_t n)
+{
+	volatile uint8_t *p = (volatile uint8_t *)v;
+	while (n--)
+		*p++ = 0;
+}