minimize SD card reads when using SDCARD_SORT_ALPHA (Re-ARM board)

Because of the Re-ARM card's pinout there is only one SPI connected to
the RepRap Discount Full Graphic LCD display.  The LCD responds to ANY
SCK transitions no matter if it's enable is inactive.  The result is
garbage (usually bars) on the LCD display whenever there is SD card
activity.

This code minimizes this by only accessing the SD card when changing
directory levels if :
SDCARD_SORT_ALPHA is enabled
SDSORT_USES_RAM is true
SDSORT_CACHE_NAMES is true

The code changes result in file names being pulled from the ALPHA SORT
memory array rather than the SD card.

The code also gives the file count and file index functions their own
variables.  When they shared a common variable the index function
sometimes resulted in the file count being short by 1.

=======================================================================

cardreader.cpp & pins_RAMPS_RE_ARM.h changes

Added another condition to cardreader.cpp to enable getting file names
only from RAM.

pins_RAMPS_RE_ARM.h :
Added comments about the SD card accesses and the LCD display
Combined all versions into this one.

Marlin/cardreader.cpp

@@ -74,9 +74,12 @@ char *createFilename(char *buffer, const dir_t &p) { //buffer > 12characters
 /**
  * Dive into a folder and recurse depth-first to perform a pre-set operation lsAction:
  *   LS_Count       - Add +1 to nrFiles for every file within the parent
- *   LS_GetFilename - Get the filename of the file indexed by nrFiles
+ *   LS_GetFilename - Get the filename of the file indexed by nrFile_index
  *   LS_SerialPrint - Print the full path and size of each file to serial output
  */
+ 
+uint16_t nrFile_index;
+
 void CardReader::lsDive(const char *prepend, SdFile parent, const char * const match/*=NULL*/) {
   dir_t p;
   uint8_t cnt = 0;
@@ -130,7 +133,7 @@ void CardReader::lsDive(const char *prepend, SdFile parent, const char * const m
 
       if (!filenameIsDir && (p.name[8] != 'G' || p.name[9] == '~')) continue;
 
-      switch (lsAction) {
+      switch (lsAction) {  // 1 based file count
         case LS_Count:
           nrFiles++;
           break;
@@ -148,7 +151,7 @@ void CardReader::lsDive(const char *prepend, SdFile parent, const char * const m
           if (match != NULL) {
             if (strcasecmp(match, filename) == 0) return;
           }
-          else if (cnt == nrFiles) return;
+          else if (cnt == nrFile_index) return;  // 0 based index
           cnt++;
           break;
       }
@@ -596,7 +599,7 @@ void CardReader::getfilename(uint16_t nr, const char * const match/*=NULL*/) {
   #endif // SDSORT_CACHE_NAMES
   curDir = &workDir;
   lsAction = LS_GetFilename;
-  nrFiles = nr;
+  nrFile_index = nr;
   curDir->rewind();
   lsDive("", *curDir, match);
 }
@@ -861,6 +864,13 @@ void CardReader::updir() {
 
 #endif // SDCARD_SORT_ALPHA
 
+#if (ENABLED(SDCARD_SORT_ALPHA) && SDSORT_USES_RAM && SDSORT_CACHE_NAMES)  
+ // if true - don't need to access the SD card for file names
+  uint16_t CardReader::get_num_Files() {return nrFiles;}
+#else
+  uint16_t CardReader::get_num_Files() {return getnrfilenames(); } 
+#endif
+
 void CardReader::printingHasFinished() {
   stepper.synchronize();
   file.close();

Marlin/cardreader.h

@@ -69,6 +69,8 @@ public:
   void updir();
   void setroot();
 
+  uint16_t get_num_Files();
+  
   #if ENABLED(SDCARD_SORT_ALPHA)
     void presort();
     void getfilename_sorted(const uint16_t nr);

Marlin/ultralcd.cpp

@@ -3507,7 +3507,7 @@ void kill_screen(const char* lcd_msg) {
     void lcd_sdcard_menu() {
       ENCODER_DIRECTION_MENUS();
       if (!lcdDrawUpdate && !lcd_clicked) return; // nothing to do (so don't thrash the SD card)
-      const uint16_t fileCnt = card.getnrfilenames();
+      const uint16_t fileCnt = card.get_num_Files();
       START_MENU();
       MENU_BACK(MSG_MAIN);
       card.getWorkDirName();
@@ -3520,6 +3520,7 @@ void kill_screen(const char* lcd_msg) {
         MENU_ITEM(function, LCD_STR_FOLDER "..", lcd_sd_updir);
       }
 
+      if (fileCnt) {
         for (uint16_t i = 0; i < fileCnt; i++) {
           if (_menuLineNr == _thisItemNr) {
             const uint16_t nr =
@@ -3543,6 +3544,7 @@ void kill_screen(const char* lcd_msg) {
             MENU_ITEM_DUMMY();
           }
         }
+      }
       END_MENU();
     }