tools: kwboot: Handle EINTR in kwboot_tty_recv()

The select() and read() syscalls may be interrupted. Handle EINTR and
retry them.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/tools/kwboot.c b/tools/kwboot.c
index 8b748f0..fca1c73 100644
--- a/tools/kwboot.c
+++ b/tools/kwboot.c
@@ -409,15 +409,19 @@
 
 	do {
 		nfds = select(fd + 1, &rfds, NULL, NULL, &tv);
-		if (nfds < 0)
+		if (nfds < 0 && errno == EINTR)
+			continue;
+		else if (nfds < 0)
 			goto out;
-		if (!nfds) {
+		else if (!nfds) {
 			errno = ETIMEDOUT;
 			goto out;
 		}
 
 		n = read(fd, buf, len);
-		if (n <= 0)
+		if (n < 0 && errno == EINTR)
+			continue;
+		else if (n <= 0)
 			goto out;
 
 		buf = (char *)buf + n;