native: increase rtc realism

Prevent rtc enabling when not initialized.

cpu/native/rtc/posix-rtc.c

@@ -30,17 +30,24 @@
 #include "native_internal.h"
 
 static int native_rtc_enabled;
+static int native_rtc_initialized;
 
 void rtc_init(void)
 {
     native_rtc_enabled = 0;
+    native_rtc_initialized = 1;
     printf("native rtc initialized\n");
 }
 
 void rtc_enable(void)
 {
     DEBUG("rtc_enable\n");
-    native_rtc_enabled = 1;
+    if (native_rtc_initialized == 1) {
+        native_rtc_enabled = 1;
+    }
+    else {
+        DEBUG("rtc not initialized, not enabling\n");
+    }
 }
 
 void rtc_disable(void)