sandbox: Restore blocking I/O on exit

At present sandbox sets non-blocking I/O as soon as any input is read
from the terminal. However it does not restore the previous state on
exit. Fix this and drop the old os_read_no_block() function.

This means that we always enable blocking I/O in sandbox (if input is a
terminal) whereas previously it would only happen on the first call to
tstc() or getc(). However, the difference is likely not important.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index 9e9bf3e..4a05ea4 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -126,7 +126,7 @@
 	if (next_index == serial_buf_read)
 		return 1;	/* buffer full */
 
-	count = os_read_no_block(0, &serial_buf[serial_buf_write], 1);
+	count = os_read(0, &serial_buf[serial_buf_write], 1);
 	if (count == 1)
 		serial_buf_write = next_index;