Checking if /var/log is writable before trying to copy live.log to th…

…e newly mounted root filesystem (Closes: #516914).

When using the "exposedroot skipunion" options, then /var/log may not be writable at this time, resulting in an error message during bootup.

scripts/live

@@ -1585,5 +1585,5 @@ mountroot ()
 	exec 1>&6 6>&-
 	exec 2>&7 7>&-
 	kill ${tailpid}
-	cp live.log "${rootmnt}/var/log/"
+	[ -w "${rootmnt}/var/log/" ] && cp live.log "${rootmnt}/var/log/" 2>/dev/null
 }