Fix logdisk

The code was broken in a subtle but crucial way: storage idle
callbacks are oneshot so after the first flush everything would
stay in the buffer forever and would never be written to the disk
thus resulting into many events being lost. This changed correctly
registers the idle callback each time the buffer is not empty.
Note that the idle storage code checks if a callback
has is in the queue already so we don't register twice.

Change-Id: Ifdf331d4b757e05b8a6902bf5926cbc7689f5109

apps/main.c

@@ -489,10 +489,6 @@ static void init(void)
 #endif
 #endif
 
-#ifdef ROCKBOX_HAS_LOGDISKF
-    init_logdiskf();
-#endif
-
 #if CONFIG_RTC
     rtc_init();
 #endif