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/uart_driver.h

@@ -0,0 +1,107 @@
+/* ----------------------------------------------------------------------------
+ *         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 UART_DRIVER_H
+#define UART_DRIVER_H
+#include "sam.h"
+#include <stdbool.h>
+#include <stdio.h>
+
+#define PINMUX_UNUSED 0xFFFFFFFF
+#define GCLK_ID_SERCOM0_CORE 0x14
+
+/* SERCOM UART available pad settings */
+enum uart_pad_settings {
+    UART_RX_PAD0_TX_PAD2 = SERCOM_USART_CTRLA_RXPO(0) | SERCOM_USART_CTRLA_TXPO(1),
+    UART_RX_PAD1_TX_PAD2 = SERCOM_USART_CTRLA_RXPO(1) | SERCOM_USART_CTRLA_TXPO(1),
+    UART_RX_PAD2_TX_PAD0 = SERCOM_USART_CTRLA_RXPO(2),
+    UART_RX_PAD3_TX_PAD0 = SERCOM_USART_CTRLA_RXPO(3),
+    UART_RX_PAD1_TX_PAD0 = SERCOM_USART_CTRLA_RXPO(1),
+    UART_RX_PAD3_TX_PAD2 = SERCOM_USART_CTRLA_RXPO(3) | SERCOM_USART_CTRLA_TXPO(1),
+};
+
+/**
+ * \brief Gets the index of the provided SERCOM instance
+ *
+ * \param Pointer to SERCOM instance
+ * \return Index of the SERCOM module
+ */
+uint32_t uart_get_sercom_index(Sercom *sercom_instance);
+
+/**
+ * \brief Initializes the UART
+ *
+ * \param Pointer to SERCOM instance
+ * \param Baud value corresponding to the desired baudrate
+ * \param SERCOM pad settings
+ */
+void uart_basic_init(Sercom *sercom, uint16_t baud_val, enum uart_pad_settings pad_conf);
+
+/**
+ * \brief Disables UART interface
+ *
+ * \param Pointer to SERCOM instance
+ */
+void uart_disable(Sercom *sercom);
+
+/**
+ * \brief Sends a single byte through UART interface
+ *
+ * \param Pointer to SERCOM instance
+ * \param Data to send
+ */
+void uart_write_byte(Sercom *sercom, uint8_t data);
+
+/**
+ * \brief Reads a single character from UART interface
+ *
+ * \param Pointer to SERCOM instance
+ * \return Data byte read
+ */
+uint8_t uart_read_byte(Sercom *sercom);
+
+/**
+ * \brief Sends buffer on UART interface
+ *
+ * \param Pointer to SERCOM instance
+ * \param Pointer to data to send
+ * \param Number of bytes to send
+ */
+void uart_write_buffer_polled(Sercom *sercom, uint8_t *ptr, uint16_t length);
+
+/**
+ * \brief Reads data on UART interface
+ *
+ * \param Pointer to SERCOM instance
+ * \param Pointer to store read data
+ * \param Number of bytes to read
+ */
+void uart_read_buffer_polled(Sercom *sercom, uint8_t *ptr, uint16_t length);
+
+#endif