vtxprintf: Introduce vtxdprintf for the ease of closures

It was suggested to eliminate the lock for sprintf. One way to do it is
to make the fake tx_byte into a closure. This patch allows it.

It's a bit tricky since we need to preserve compatibility with romcc.

Change-Id: I877ef0cef54dcbb0589fe858c485f76f3dd27ece
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/4287
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/console/vtxprintf.h

@@ -36,4 +36,8 @@ typedef __builtin_va_list	va_list;
 
 int vtxprintf(void (*tx_byte)(unsigned char byte), const char *fmt, va_list args);
 
+#if !defined (__ROMCC__) && !defined (__SMM__)
+int vtxdprintf(void (*tx_byte)(unsigned char byte, void *data), const char *fmt, va_list args, void *data);
+#endif
+
 #endif