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: Tom Rix <Tom.Rix@windriver.com>

include/serial.h

diff --git a/include/serial.h b/include/serial.h
index bbda3f0..43451a3 100644
--- a/include/serial.h
+++ b/include/serial.h
@@ -66,4 +66,10 @@
 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