sandbox: Improve sandbox serial port keyboard interface

Implements the tstc() interface for the serial driver.  Multiplexing
the console between the serial port and a keyboard uses a polling
method of checking if characters are available; this means that the
serial console must be non-blocking when attempting to read
characters.

Signed-off-by: Taylor Hutt <thutt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>

include/os.h

@@ -39,6 +39,16 @@ struct sandbox_state;
  */
 ssize_t os_read(int fd, void *buf, size_t count);
 
+/**
+ * Access to the OS read() system call with non-blocking access
+ *
+ * \param fd	File descriptor as returned by os_open()
+ * \param buf	Buffer to place data
+ * \param count	Number of bytes to read
+ * \return number of bytes read, or -1 on error
+ */
+ssize_t os_read_no_block(int fd, void *buf, size_t count);
+
 /**
  * Access to the OS write() system call
  *