boards/samd21-arduino-bootloader: add common board module

This module implements the 2 functions called when requesting a board 'reset in application' and 'board reset in bootloader' actions.
This module will also configure the behaviour of bossa flasher and has a dependency on USBUS CDC ACM module for providing STDIO over USB

boards/common/samd21-arduino-bootloader/Makefile

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+MODULE = boards_common_samd21-arduino-bootloader
+
+include $(RIOTBASE)/Makefile.base

boards/common/samd21-arduino-bootloader/Makefile.dep

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+# Provide stdio over USB by default
+# This is a temporary solution until management of stdio is correctly
+# handled by the build system
+DEFAULT_MODULE += stdio_cdc_acm
+
+# Default auto-initialization of usbus for stdio other USB
+USEMODULE += auto_init_usbus
+
+# This boards requires support for Arduino bootloader.
+USEMODULE += usb_board_reset
+USEMODULE += boards_common_samd21-arduino-bootloader
+
+FEATURES_REQUIRED += bootloader_arduino

boards/common/samd21-arduino-bootloader/Makefile.include

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+# setup the flash tool used
+# Bossa is the default programmer
+PROGRAMMER ?= bossa
+
+ifeq ($(PROGRAMMER),bossa)
+  # by default, we use BOSSA to flash this board and take into account the
+  # preinstalled Arduino bootloader. ROM_OFFSET skips the space taken by
+  # such bootloader.
+  ROM_OFFSET ?= 0x2000
+  BOSSA_ARDUINO_PREFLASH = yes
+  PREFLASH_DELAY = 1
+
+  ifneq (,$(filter reset flash flash-only, $(MAKECMDGOALS)))
+    # By default, add 2 seconds delay before opening terminal: this is required
+    # when opening the terminal right after flashing. In this case, the stdio
+    # over USB needs some time after reset before being ready.
+    TERM_DELAY ?= 2
+    TERMDEPS += term-delay
+  endif
+
+  include $(RIOTMAKE)/tools/bossa.inc.mk
+endif
+
+term-delay:
+	sleep $(TERM_DELAY)
+
+TESTRUNNER_CONNECT_DELAY ?= $(TERM_DELAY)
+$(call target-export-variables,test,TESTRUNNER_CONNECT_DELAY)
+
+# Add the samd21-arduino-bootloader directory to the build
+DIRS += $(RIOTBOARD)/common/samd21-arduino-bootloader

boards/common/samd21-arduino-bootloader/reset.c

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+/*
+ * Copyright (C)  2019 Inria
+ *                2019 Kees Bakker, SODAQ
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     boards_common
+ * @{
+ * @file
+ * @brief       Board common implementations for managing an Arduino bootloader
+ *
+ * @author      Alexandre Abadie <alexandre.abadie@inria.fr>
+ * @author      Kees Bakker <kees@sodaq.com>
+ *
+ * @}
+ */
+
+#define USB_H_USER_IS_RIOT_INTERNAL
+
+#include "usb_board_reset.h"
+
+
+#define SAMD21_DOUBLE_TAP_ADDR              (0x20007FFCUL)
+#define SAMD21_DOUBLE_TAP_MAGIC_NUMBER      (0x07738135UL)
+
+void usb_board_reset_in_bootloader(void)
+{
+    /* The Arduino bootloader checks for a magic number in SRAM to remain in
+       bootloader mode.
+       See
+       https://github.com/arduino/ArduinoCore-samd/blob/master/bootloaders/zero/board_definitions_arduino_mkr1000.h#L38
+       and
+       https://github.com/arduino/ArduinoCore-samd/blob/master/bootloaders/zero/main.c#L94
+       for implementation details. */
+    uint32_t *reset_addr = (uint32_t *)SAMD21_DOUBLE_TAP_ADDR;
+    *reset_addr = (uint32_t)SAMD21_DOUBLE_TAP_MAGIC_NUMBER;
+
+    usb_board_reset_in_application();
+}