sys/ssp: use a random value as the canary

This implements the randomization of canary values on each build as
mentioned in the comment above the STACK_CHK_GUARD macro. Since the
build system generates a new canary for each build, i.e. riotbuild.h is
modified, the entire code is rebuild on each make invocation. This is
strictly necessary as otherwise some object files may still use the old
canary value.

Implementing this properly would require generating a cryptographically
secure random value on each boot of the RIOT operating system. This is
not deemed possible on some constrained devices, e.g. due to lack of
hardware random number generators. Besides, RIOT only seems to support a
PRNG (random module) currently. While this may be implemented in the
future for some devices the changes implemented in this commit may still
be used as a fallback then.

sys/ssp/Makefile.include

@@ -1,3 +1,6 @@
+# Canary value is randomized on each new build
+CFLAGS += -DSTACK_CHK_GUARD=$(RIOTCANARY)
+
 ifneq (,$(filter ssp,$(USEMODULE)))
  CFLAGS += -fstack-protector
 endif

sys/ssp/ssp.c

@@ -22,9 +22,6 @@
 
 #include "panic.h"
 
-/* this should be randomized for each build */
-#define STACK_CHK_GUARD 0x595e9fbd94fda766
-
 uintptr_t __stack_chk_guard = (uintptr_t) STACK_CHK_GUARD;
 
 __attribute__((noreturn)) void __stack_chk_fail(void)