Make SPIFFS and LittleFS stay out of link when not needed (#6699)

* define two weak functions defaulting to no-op
redefine them to do something useful when either spiffs or littlefs are used

* noop

* single entry point for closing FSes

* rename functions, override when instanciated, add link to explanation

* spiffs: call end on destructor

cores/esp8266/FS.h

@@ -245,6 +245,13 @@ protected:
 
 } // namespace fs
 
+extern "C"
+{
+void close_all_fs(void);
+void littlefs_request_end(void);
+void spiffs_request_end(void);
+}
+
 #ifndef FS_NO_GLOBALS
 using fs::FS;
 using fs::File;