Add option to keep FS classes in namespace (#2030)

Putting `#define FS_NO_GLOBALS` before `#include <FS.h>` will disable `using` declarations for FS classes.

Author: igrr

cores/esp8266/FS.h

@@ -127,6 +127,7 @@ class FS
 
 } // namespace fs
 
+#ifndef FS_NO_GLOBALS
 using fs::FS;
 using fs::File;
 using fs::Dir;
@@ -135,7 +136,8 @@ using fs::SeekSet;
 using fs::SeekCur;
 using fs::SeekEnd;
 using fs::FSInfo;
+#endif //FS_NO_GLOBALS
 
-extern FS SPIFFS;
+extern fs::FS SPIFFS;
 
 #endif //FS_H