Add crossfade feature tag, update manual, fix crossfade WPS tag behavior

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@23610 a1c6a512-1295-4272-9138-f99709370657

apps/features.txt

@@ -42,6 +42,10 @@ button_light
 buttonlight_brightness
 #endif
 
+#if defined(HAVE_CROSSFADE)
+crossfade
+#endif
+
 #if defined(HAVE_DIRCACHE)
 dircache
 #endif

apps/gui/skin_engine/skin_tokens.c

@@ -770,10 +770,10 @@ const char *get_token_value(struct gui_wps *gwps,
             if (intval)
                 *intval = global_settings.crossfade + 1;
             snprintf(buf, buf_size, "%d", global_settings.crossfade);
-            return buf;
 #else
-            return NULL;
+            snprintf(buf, buf_size, "%d", 0);
 #endif
+            return buf;
 
         case WPS_TOKEN_REPLAYGAIN:
         {