RTC changes :

- Replace linux-time.c and win-time.c with universal rtc.c code
- Add RTC binary and 12-hour modes, century register, and alarm support
- Cleaned up nvr.c a bit
- Fix handling of update ended flag
- Add ability to disable host clock synchronisation
Changes from Battler and Mahod.

src/nvr.h

@@ -1,5 +1,7 @@
 void nvr_init();
 
+extern int enable_sync;
+
 extern int nvr_dosave;
 
 void time_get(char *nvrram);