usbdebug: Fix boards without EARLY_CBMEM_INIT

The main usbdebug file lib/usbdebug.c was removed from romstage
build with commit f8bf5a10 but the chipset-specific parts were not,
leading to unresolved symbol errors for AMD platforms.

Add a silent Kconfig variable USBDEBUG_IN_ROMSTAGE for convenient
use of this feature.

Change-Id: I0cd3fccf2612cf08497aa5c3750c89bf43ff69be
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3983
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/console/console.c

@@ -119,7 +119,7 @@ void console_init(void)
 #if CONFIG_SPKMODEM
 	spkmodem_init();
 #endif
-#if CONFIG_USBDEBUG && CONFIG_EARLY_CBMEM_INIT && !defined(__BOOT_BLOCK__)
+#if CONFIG_USBDEBUG_IN_ROMSTAGE && !defined(__BOOT_BLOCK__)
 	usbdebug_init();
 #endif