hosted: call 'sync' before poweroff

Might be needed to avoid filesystem corruption with buggy OF kernels.

Change-Id: I589eec6c1536beccf849716f11204b4b83a942a1

firmware/target/hosted/system-hosted.c

@@ -90,6 +90,7 @@ static void sig_handler(int sig, siginfo_t *siginfo, void *context)
 void power_off(void)
 {
     backlight_hw_off();
+    sync();
     system("/sbin/poweroff");
     while (1);       /* halt */
 }