Add example to update ESP32 via SD card (#628)

* Add example to update ESP32 via SD card

* Delete update.bin from sd upon finishing update

* remove precompiled binary from example

* Check whether update.bin is a file

libraries/Update/examples/SD_Update/SD_Update.ino

+/* 
+ Name:      SD_Update.ino
+ Created:   12.09.2017 15:07:17
+ Author:    Frederik Merz <frederik.merz@novalight.de>
+ Purpose:   Update firmware from SD card
+
+ Steps:
+   1. Flash this image to the ESP32 an run it
+   2. Copy update.bin to a SD-Card, you can basically
+      compile this or any other example
+      then copy and rename the app binary to the sd card root
+   3. Connect SD-Card as shown in SD_MMC example, 
+      this can also be adapted for SPI
+   3. After successfull update and reboot, ESP32 shall start the new app
+*/
+
+#include <Update.h>
+#include <FS.h>
+#include <SD_MMC.h>
+
+// perform the actual update from a given stream
+void performUpdate(Stream &updateSource, size_t updateSize) {
+   if (Update.begin(updateSize)) {      
+      size_t written = Update.writeStream(updateSource);
+      if (written == updateSize) {
+         Serial.println("Written : " + String(written) + " successfully");
+      }
+      else {
+         Serial.println("Written only : " + String(written) + "/" + String(updateSize) + ". Retry?");
+      }
+      if (Update.end()) {
+         Serial.println("OTA done!");
+         if (Update.isFinished()) {
+            Serial.println("Update successfully completed. Rebooting.");
+         }
+         else {
+            Serial.println("Update not finished? Something went wrong!");
+         }
+      }
+      else {
+         Serial.println("Error Occurred. Error #: " + String(Update.getError()));
+      }
+
+   }
+   else
+   {
+      Serial.println("Not enough space to begin OTA");
+   }
+}
+
+// check given FS for valid update.bin and perform update if available
+void updateFromFS(fs::FS &fs) {
+   File updateBin = fs.open("/update.bin");
+   if (updateBin) {
+      if(updateBin.isDirectory()){
+         Serial.println("Error, update.bin is not a file");
+         updateBin.close();
+         return;
+      }
+
+      size_t updateSize = updateBin.size();
+
+      if (updateSize > 0) {
+         Serial.println("Try to start update");
+         performUpdate(updateBin, updateSize);
+      }
+      else {
+         Serial.println("Error, file is empty");
+      }
+
+      updateBin.close();
+    
+      // whe finished remove the binary from sd card to indicate end of the process
+      fs.remove("/update.bin");      
+   }
+   else {
+      Serial.println("Could not load update.bin from sd root");
+   }
+}
+
+void setup() {
+   uint8_t cardType;
+   Serial.begin(115200);
+   Serial.println("Welcome to the SD-Update example!");
+
+   // You can uncomment this and build again
+   // Serial.println("Update successfull");
+
+   //first init and check SD card
+   if (!SD_MMC.begin()) {
+      Serial.println("Card Mount Failed");
+      goto end;
+   }
+
+   cardType = SD_MMC.cardType();
+
+   if (cardType == CARD_NONE) {
+      Serial.println("No SD_MMC card attached");
+      goto end;
+   }
+
+   updateFromFS(SD_MMC);
+
+end: 
+   delay(1000);
+   ESP.restart();
+}
+
+//will not be reached
+void loop() {
+  
+}