Don't prompt for date/time twice when booting a machine with no #r/rtc (e.g. raspberry pi).

Reference: /n/sources/patch/applied/boot-settime-once
Date: Thu Aug 16 22:05:20 CES 2012
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/boot/settime.c	Thu Aug 16 22:04:14 2012
+++ /sys/src/9/boot/settime.c	Thu Aug 16 22:04:11 2012
@@ -12,12 +12,13 @@
 settime(int islocal, int afd, char *rp)
 {
 	int n, f;
-	int timeset;
+	static int timeset;
 	Dir dir[2];
 	char timebuf[64];
 
+	if(timeset)
+		return;
 	print("time...");
-	timeset = 0;
 	if(islocal){
 		/*
 		 *  set the time from the real time clock