sdcard: add support for fatfs

sdcard/README.md

@@ -0,0 +1,20 @@
+# SPI sdcard/mmc driver
+
+This package provides the driver for sdcard/mmc with SPI connection.  
+`examples/sdcard/console` shows a low-level access example.  
+`examples/sdcard/tinyfs` shows an example of using fatfs to read FAT32.  
+
+If you use this package, you need to set `default-stack-size` in `targets/*.json`.  
+For example, `targets/wioterminal.json` has the following configuration.  
+
+```
+{
+    "inherits": ["atsamd51p19a"],
+    "build-tags": ["wioterminal"],
+    "flash-1200-bps-reset": "true",
+    "flash-method": "msd",
+    "msd-volume-name": "Arduino",
+    "msd-firmware-name": "firmware.uf2",
+    "default-stack-size": 2048
+}
+```