x86: expose console_tx_flush in romstage

The vboot module relied on being able to flush the console
after it called vtxprintf() from its log wrapper function.
Expose the console_tx_flush() function in romstage so the
vboot module can ensure messages are flushed.

Change-Id: I578053df4b88c2068bd9cc90eea5573069a0a4e8
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2882
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/x86/lib/romstage_console.c

@@ -54,7 +54,7 @@ void console_tx_byte(unsigned char byte)
 #endif
 }
 
-static void console_tx_flush(void)
+void console_tx_flush(void)
 {
 #if CONFIG_CONSOLE_SERIAL8250MEM
 	uart8250_mem_tx_flush(CONFIG_OXFORD_OXPCIE_BASE_ADDRESS + 0x1000);