Use lwm2m_seed() to get the seed

The lwm2m_seed() function is used to obtain a seed for random number
generation.
The implementation of the lwm2m_seed() function is up to the user.

core/liblwm2m.c

@@ -68,7 +68,7 @@ lwm2m_context_t * lwm2m_init(void * userData)
     {
         memset(contextP, 0, sizeof(lwm2m_context_t));
         contextP->userData = userData;
-        srand((int)lwm2m_gettime());
+        srand((int)lwm2m_seed());
         contextP->nextMID = rand();
     }
 

examples/shared/platform.c

@@ -67,6 +67,17 @@ time_t lwm2m_gettime(void)
     return time(NULL);
 }
 
+int lwm2m_seed(void)
+{
+    /* 
+     * Return a seed for random number generation, the seed must be a 
+     * different number at every boot and unpredictable, time(NULL) may not be 
+     * a reliable source as a seed.
+     * See: https://github.com/eclipse/wakaama/pull/711
+     */
+    return time(NULL);
+}
+
 void lwm2m_printf(const char * format, ...)
 {
     va_list ap;

include/liblwm2m.h

@@ -118,6 +118,8 @@ int lwm2m_strcasecmp(const char * str1, const char * str2);
 // In case of error, this must return a negative value.
 // Per POSIX specifications, time_t is a signed integer.
 time_t lwm2m_gettime(void);
+// Get a seed (which must not repeat when the device reboots) for generating a random number
+int lwm2m_seed(void);
 
 #ifdef LWM2M_WITH_LOGS
 // Same usage as C89 printf()