Made srand(...) and rand() platform independent

The srand(...) and rand() functions are not available on all embedded
platforms, so they were replaced with lwm2m_srand(...) and lwm2m_rand()
to make these calls platform independent.

core/liblwm2m.c

@@ -68,8 +68,8 @@ lwm2m_context_t * lwm2m_init(void * userData)
     {
         memset(contextP, 0, sizeof(lwm2m_context_t));
         contextP->userData = userData;
-        srand((int)lwm2m_gettime());
-        contextP->nextMID = rand();
+        lwm2m_srand((int)lwm2m_gettime());
+        contextP->nextMID = lwm2m_rand();
     }
 
     return contextP;

examples/shared/platform.c

@@ -67,6 +67,16 @@ time_t lwm2m_gettime(void)
     return time(NULL);
 }
 
+void lwm2m_srand(unsigned int seed)
+{
+    srand(seed);
+}
+
+int lwm2m_rand()
+{
+    return rand();
+}
+
 void lwm2m_printf(const char * format, ...)
 {
     va_list ap;

include/liblwm2m.h

@@ -118,6 +118,10 @@ 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);
+// Initialize random number generator
+void lwm2m_srand(unsigned int seed);
+// Get a random number
+int lwm2m_rand(void);
 
 #ifdef LWM2M_WITH_LOGS
 // Same usage as C89 printf()