Prevent SPI scanning if SD card fails to mount

Fix #758

Author: nseidle

Firmware/RTK_Surveyor/Begin.ino

@@ -363,6 +363,9 @@ void beginBoard()
 
 void beginSD()
 {
+    if(sdCardForcedOffline == true)
+        return;
+
     bool gotSemaphore;
 
     online.microSD = false;
@@ -447,6 +450,8 @@ void beginSD()
                     systemPrintln("SD init failed - using SPI and SdFat. Is card formatted?");
                     digitalWrite(pin_microSD_CS, HIGH); // Be sure SD is deselected
 
+                    sdCardForcedOffline = true; //Prevent future scans for SD cards
+
                     // Check reset count and prevent rolling reboot
                     if (settings.resetCount < 5)
                     {

Firmware/RTK_Surveyor/RTK_Surveyor.ino

@@ -720,6 +720,9 @@ unsigned long shutdownNoChargeTimer = 0;
 
 RtkMode_t rtkMode; // Mode of operation
 
+bool sdCardForcedOffline = false; //Goes true if a isPresent() test passes, but then sdFat fails to mount SD card.
+//See issue: https://github.com/sparkfun/SparkFun_RTK_Firmware/issues/758
+
 //-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 
 #define DEAD_MAN_WALKING_ENABLED    0