Add filesystem example and update filesystem api

Arduino_package/hardware/libraries/FileSystem/examples/GetFreeAndUseSpace/GetFreeAndUseSpace.ino

@@ -0,0 +1,37 @@
+/*
+ This sketch shows how to open/close file and perform read/write to it.
+
+ Example guide:
+ https://www.amebaiot.com/en/
+ */
+
+#include "AmebaFatFS.h"
+
+uint64_t device_used_bytes;
+uint64_t device_free_bytes;
+
+uint16_t device_used_Mbytes;
+uint16_t device_free_Mbytes;
+
+AmebaFatFS fs;
+void setup()
+{
+    fs.begin();
+    device_used_bytes = fs.get_used_space();
+    device_used_Mbytes = (uint16_t)(device_used_bytes / 1048576);    // convert bytes to Mbytes /(1024*1024)
+
+    device_free_bytes = fs.get_free_space();
+    device_free_Mbytes = (uint16_t)(device_free_bytes / 1048576);    // convert bytes to Mbytes /(1024*1024)
+
+    printf("Use space on SD card: %llu bytes\r\n", device_used_bytes);
+    printf("Use space on SD card: %u Mbytes\r\n", device_used_Mbytes);
+
+    printf("Free space on SD card: %llu bytes\r\n", device_free_bytes);
+    printf("Free space on SD card: %u Mbytes\r\n", device_free_Mbytes);
+    fs.end();
+}
+
+void loop()
+{
+    delay(1000);
+}

Arduino_package/hardware/libraries/FileSystem/src/AmebaFatFS.cpp

@@ -329,3 +329,13 @@ int AmebaFatFS::getAttribute(char *path, unsigned char *attr)
     } while (0);
     return (-res);
 }
+
+long long int AmebaFatFS::get_free_space(void)
+{
+    return fatfs_get_free_space_byte();
+}
+
+long long int AmebaFatFS::get_used_space(void)
+{
+    return fatfs_get_used_space_byte();
+}

Arduino_package/hardware/libraries/FileSystem/src/AmebaFatFS.h

@@ -47,7 +47,8 @@ class AmebaFatFS {
     int setLastModTime(char *path, uint16_t year, uint16_t month, uint16_t date, uint16_t hour, uint16_t minute, uint16_t second);
 
     int status(void);
-
+    long long int get_free_space(void);
+    long long int get_used_space(void);
 private:
     int getAttribute(char *path, unsigned char *attr);