Add UF2 bootloader for USB firmware flashing

Vendored and trimmed from microsoft/uf2-samdx1, adapted to the actual
ATSAMD21G17D (128 KiB flash / 16 KiB RAM), fixing the flash/RAM-size
mismatch that the previously commented-out SAM-BA bootloader target
had (it assumed a 256 KiB SAMD21G18A). Builds as a standalone
PlatformIO environment (bootloader/platformio.ini), no python2/make
dependency. Compiles clean, fits in the 8 KiB bootloader region
(7292/8192 bytes). Not yet flashed/verified on real hardware.

bootloader/inc/usart_sam_ba.h

@@ -0,0 +1,158 @@
+/* ----------------------------------------------------------------------------
+ *         SAM Software Package License
+ * ----------------------------------------------------------------------------
+ * Copyright (c) 2011-2014, Atmel Corporation
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following condition is met:
+ *
+ * Redistributions of source code must retain the above copyright notice,
+ * this list of conditions and the disclaimer below.
+ *
+ * Atmel's name may not be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * DISCLAIMER: THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
+ * DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA,
+ * OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
+ * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ * ----------------------------------------------------------------------------
+ */
+
+#ifndef _USART_SAM_BA_H_
+#define _USART_SAM_BA_H_
+
+#include "stdbool.h"
+#include "stdint.h"
+
+/* USART buffer size (must be a power of two) */
+#define USART_BUFFER_SIZE 128
+
+/* Define the default time-out value for USART. */
+#define USART_DEFAULT_TIMEOUT 1000
+
+/* Xmodem related defines */
+/* CRC16  polynomial */
+#define CRC16POLY 0x1021
+
+#define SHARP_CHARACTER '#'
+
+/* X/Ymodem protocol: */
+#define SOH 0x01
+//#define STX                    0x02
+#define EOT 0x04
+#define ACK 0x06
+#define NAK 0x15
+#define CAN 0x18
+#define ESC 0x1b
+
+#define PKTLEN_128 128
+
+/**
+ * \brief Open the given USART
+ */
+void usart_open(void);
+
+/**
+ * \brief Stops the USART
+ */
+void usart_close(void);
+
+/**
+ * \brief Puts a byte on usart line
+ *
+ * \param value      Value to put
+ *
+ * \return \c 1 if function was successfully done, otherwise \c 0.
+ */
+int usart_putc(int value);
+
+/**
+ * \brief Waits and gets a value on usart line
+ *
+ * \return value read on usart line
+ */
+int usart_getc(void);
+
+/**
+ * \brief Returns true if the SAM-BA Uart received the sharp char
+ *
+ * \return Returns true if the SAM-BA Uart received the sharp char
+ */
+int usart_sharp_received(void);
+
+/**
+ * \brief This function checks if a character has been received on the usart
+ * line
+ *
+ * \return \c 1 if a byte is ready to be read.
+ */
+bool usart_is_rx_ready(void);
+
+/**
+ * \brief Gets a value on usart line
+ *
+ * \return value read on usart line
+ */
+int usart_readc(void);
+
+/**
+ * \brief Send buffer on usart line
+ *
+ * \param data pointer
+ * \param number of data to send
+ * \return number of data sent
+ */
+uint32_t usart_putdata(void const *data, uint32_t length); // Send given data (polling)
+
+/**
+ * \brief Gets data from usart line
+ *
+ * \param data pointer
+ * \param number of data to get
+ * \return value read on usart line
+ */
+uint32_t usart_getdata(void *data, uint32_t length); // Get data from comm. device
+
+/**
+ * \brief Send buffer on usart line using Xmodem protocol
+ *
+ * \param data pointer
+ * \param number of data to send
+ * \return number of data sent
+ */
+uint32_t
+usart_putdata_xmd(void const *data,
+                  uint32_t length); // Send given data (polling) using xmodem (if necessary)
+
+/**
+ * \brief Gets data from usart line using Xmodem protocol
+ *
+ * \param data pointer
+ * \param number of data to get
+ * \return value read on usart line
+ */
+uint32_t
+usart_getdata_xmd(void *data,
+                  uint32_t length); // Get data from comm. device using xmodem (if necessary)
+
+/**
+ * \brief Gets data from usart line using Xmodem protocol
+ *
+ * \param data pointer
+ * \param number of data to get
+ * \return value read on usart line
+ */
+uint16_t add_crc(uint8_t ch, unsigned short crc0);
+
+uint8_t getPacket(uint8_t *pData, uint8_t sno);
+
+#endif // _USART_SAM_BA_H_