NuMaker SD-File-System with SD mode

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+# Example for SD card backed file system
+
+This example is cloned from the [FAT file system example](https://github.com/armmbed/mbed-os-example-fat-filesystem) released by ARM mbed team and
+is modified to use the [SD block device](https://github.com/OpenNuvoton/NuMaker-mbed-SD-driver)
+running in SD bus mode rather than SPI bus mode to back the file system.