add README file

this describes the project, how to compile, and how to flash the
USB DFU bootloader

Change-Id: If884bca373aab10fe74d16f42b79dd6685eeabcf

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+This is an implementation of the DFU mode of the [USB DFU Device Class Specification](https://usb.org/document-library/device-firmware-upgrade-11-new-version-31-aug-2004) for the Microchip SAM D5x/E5x micro-controller.
+It is meant to be used as bootloader to allow flashing the main application over USB.
+
+The code has been developed for the Microchip [SAM E54 Xplained Pro](https://www.microchip.com/DevelopmentTools/ProductDetails/PartNo/ATSAME54-XPRO) development board using a [SAM E54](https://www.microchip.com/wwwproducts/en/ATSAME54P20A) micro-controller.
+It should work on any chip of the SAM D5x/E5x device family by replacing the corresponding device-specific definitions (usually including the chip name in the file name).
+
+The code uses the [Atmel START](https://start.atmel.com/) ASFv4 library.
+
+USB DFU
+=======
+
+This implementation support the following USB DFU capabilities:
+
+* can download: allowing to download the code over USB on the device (enabled per default)
+* manifestation tolerant: allowing to download after a previous download (disabled per default)
+* will detach: forcing the device the reset after a download, else it wit for a USB reset (enable per default)
+
+Set the corresponding attributes in the 'DFUD_IFACE_DESCB' macro definition in the 'usb/class/dfu/device/dfudf_desc.h' file.
+
+Compiling
+=========
+
+Use the 'Makefile' script to compile the source code using the ARM none EABI GCC cross-cimpilig toolchain:
+```
+cd gcc
+make
+```
+
+The resulting firmware binary is `AtmelStart.bin`.
+
+Flashing
+========
+
+To flash the bootloader you can either use the [edbg tool](https://github.com/ataradov/edbg) over the EDBG interface of the SAM E54 Xplained Pro development board, or OpenICD with any SWJ adapter.
+
+The USB DFU bootloader should be flashed in a protected area of the flash memory to prevent for erasing it, as specified in data sheet section 25.6.2 Memory Organization.
+The bootloader size is configured in the NVM user configuration BOOTPROT field, as specified in data sheet section 25.6.9 NVM User Configuration.
+The bit position of the BOOTPROT field is documented in data sheet section 9.4 NVM User Page Mapping.
+We need to reserve as least 2 pages of 8192 bytes for the bootloader since it is a bit larger than 10 KB (e.g. over 8 KB).
+This setting will also tell the bootloader where to flash the application firmware to (e.g. after the bootloader reserved space).
+The LED will blink once per second if this size is not set.
+
+EDBG
+----
+
+To flash the USB DFU bootloader, perform the following actions:
+* remove reserved bootloader space so we can erase it: `edbg --target atmel_cm4v2 --fuse wv,29:26,15`
+* erase the whole flash: `edbg --target atmel_cm4v2 --fuse v,29:26,15 --erase`
+* program the bootloader: `edbg --target atmel_cm4v2 --fuse v,29:26,15 --program --verify --file AtmelStart.bin`
+* reserve bootloader space: `edbg --target atmel_cm4v2 --fuse wv,29:26,13`