tools: kwboot: properly quit when read() returns 0

When kwboot is attached to a terminal which disappears such as one
connected via an unplugged USB cable, read() returns 0, making kwboot
loop until a key is pressed in the terminal. The only case where read()
may return 0 here is when the terminal is closed anyway, so let's
properly handle this one and report is similar to other errors.
Signed-off-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NStefan Roese <sr@denx.de>

tools/kwboot.c

@@ -182,7 +182,7 @@ kwboot_tty_recv(int fd, void *buf, size_t len, int timeo)
 		}
 
 		n = read(fd, buf, len);
-		if (n < 0)
+		if (n <= 0)
 			goto out;
 
 		buf = (char *)buf + n;
@@ -466,7 +466,7 @@ kwboot_term_pipe(int in, int out, char *quit, int *s)
 	char _buf[128], *buf = _buf;
 
 	nin = read(in, buf, sizeof(buf));
-	if (nin < 0)
+	if (nin <= 0)
 		return -1;
 
 	if (quit) {