console: Add printk helper for ChromeOS

Do not expose console_tx_flush() to ChromeOS as that function
is part of lower-level implementation.

Change-Id: I1e31662da88a60e83f8e5d307a4b53441c130aab
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5347
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/vendorcode/google/chromeos/vboot_loader.c

@@ -64,8 +64,7 @@ out:
 /* Helper routines for the vboot stub. */
 static void log_msg(const char *fmt, va_list args)
 {
-	vtxprintf(console_tx_byte, fmt, args);
-	console_tx_flush();
+	do_vtxprintf(fmt, args);
 }
 
 static void fatal_error(void)