common: Move serial_printf() to the serial header

Move this function header to serial.h since this function is clearly
related to serial. The function itself stays in console.c since we don't
have a single serial file. DM and non-DM each has a separate file so we
would have to either create a new common serial file, or repeat the
function in both serial.c and serial-uclass.c, neither of which seem
worthwhile.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/serial.h

@@ -324,4 +324,15 @@ void pl01x_serial_initialize(void);
 void pxa_serial_initialize(void);
 void sh_serial_initialize(void);
 
+/**
+ * serial_printf() - Write a formatted string to the serial console
+ *
+ * The total size of the output must be less than CONFIG_SYS_PBSIZE.
+ *
+ * @fmt: Printf format string, followed by format arguments
+ * @return number of characters written
+ */
+int serial_printf(const char *fmt, ...)
+		__attribute__ ((format (__printf__, 1, 2)));
+
 #endif