solved some compiler warnings that are now visible in arduino 1.0.

Found a couple of unused variables, that I commented.
Tried to solve the program memory warning message, and failed.

Marlin/cardreader.h

@@ -59,7 +59,7 @@ private:
   LsAction lsAction; //stored for recursion.
   int16_t nrFiles; //counter for the files in the current directory and recycled as position counter for getting the nrFiles'th name in the directory.
   char* diveDirName;
-  void lsDive(char *prepend,SdFile parent);
+  void lsDive(const char *prepend,SdFile parent);
 };