added check_stack() to determine stack usage with Contiki

tinydtls uses some stack space for HMAC calculations etc. This may
cause trouble on platforms where the stack is very small. For example,
the platform mbxxx (using an STM32 CPU) has reserved 1280 bytes for
this. During tests, I found that this is too limited for a complete
handshake. check_stack() shows that around 2 KiB should work.

debug.h

@@ -37,8 +37,27 @@
 #  define DEBUG DEBUG_PRINT
 # endif /* DEBUG */
 #include "net/ip/uip-debug.h"
-#else
+
+#ifdef CONTIKI_TARGET_MBXXX
+extern char __Stack_Init, _estack;
+
+static inline void check_stack() {
+  const char *p = &__Stack_Init;
+  while (p < &_estack && *p == 0x38) {
+    p++;
+  }
+
+  PRINTF("Stack: %d bytes used (%d free)\n", &_estack - p, p - &__Stack_Init);
+}
+#else /* CONTIKI_TARGET_MBXXX */
+static inline void check_stack() {
+}
+#endif /* CONTIKI_TARGET_MBXXX */
+#else /* WITH_CONTKI */
 #define PRINTF(...)
+
+static inline void check_stack() {
+}
 #endif
 
 struct __session_t;

dtls.c

@@ -3135,6 +3135,7 @@ handle_handshake_msg(dtls_context_t *ctx, dtls_peer_t *peer, session_t *session,
     dtls_handshake_free(peer->handshake_params);
     peer->handshake_params = NULL;
     dtls_debug("Handshake complete\n");
+    check_stack();
     peer->state = DTLS_STATE_CONNECTED;
 
     /* return here to not increase the message receive counter */