USBTTY make some function declarations easier to use.

Zoom2 needs to use these declarations and the include directory is a
better place from them than in the middle of the driver directory.
It did not make sense to create a new file for just a couple of
lines so they were appended to the serial.h
Signed-off-by: NTom Rix <Tom.Rix@windriver.com>

include/serial.h

@@ -66,4 +66,10 @@ extern void serial_stdio_init(void);
 extern int serial_assign(char * name);
 extern void serial_reinit_all(void);
 
+/* For usbtty */
+extern int usbtty_getc(void);
+extern void usbtty_putc(const char c);
+extern void usbtty_puts(const char *str);
+extern int usbtty_tstc(void);
+
 #endif