boards: Add support for the WeMos-Zero

This is an Arduino Zero clone without the embedded debugger. Thus, it will be
flashed by default using the U2F bootloader and uses by default USB CDC ACM for
STDIO.

boards/wemos-zero/Makefile

@@ -0,0 +1,6 @@
+MODULE = board
+
+DIRS += $(RIOTBOARD)/common/samd21-arduino-bootloader
+DIRS += $(RIOTBOARD)/common/arduino-zero
+
+include $(RIOTBASE)/Makefile.base

boards/wemos-zero/Makefile.dep

@@ -0,0 +1,12 @@
+include $(RIOTBOARD)/common/arduino-zero/Makefile.dep
+
+# use arduino-bootloader only if no other stdio_% other than stdio_cdc_acm
+# is requested
+ifeq (,$(filter-out stdio_cdc_acm,$(filter stdio_%,$(USEMODULE))))
+  USEMODULE += boards_common_samd21-arduino-bootloader
+endif
+
+ifneq (,$(filter boards_common_samd21-arduino-bootloader,$(USEMODULE)))
+  # setup the samd21 arduino bootloader related dependencies
+  include $(RIOTBOARD)/common/samd21-arduino-bootloader/Makefile.dep
+endif

boards/wemos-zero/Makefile.features

@@ -0,0 +1,3 @@
+include $(RIOTBOARD)/common/arduino-zero/Makefile.features
+
+FEATURES_PROVIDED += bootloader_arduino

boards/wemos-zero/Makefile.include

@@ -0,0 +1,18 @@
+PORT_LINUX ?= /dev/ttyACM0
+PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))
+
+CFLAGS += -DBOOTLOADER_UF2
+
+# setup serial terminal
+include $(RIOTMAKE)/tools/serial.inc.mk
+
+# setup the flash tool used
+ifeq ($(PROGRAMMER),jlink)
+  # in case J-Link is attached to SWD pins, use a plain CPU memory model
+  JLINK_DEVICE := atsamd21
+  include $(RIOTMAKE)/tools/jlink.inc.mk
+endif
+
+# Include all definitions for flashing with bossa other USB
+include $(RIOTBOARD)/common/samd21-arduino-bootloader/Makefile.include
+include $(RIOTBOARD)/common/arduino-zero/Makefile.include

boards/wemos-zero/doc.txt

@@ -0,0 +1,61 @@
+/**
+@defgroup    boards_wemos-zero WeMos Arduino Zero Clone
+@ingroup     boards
+@brief       Support for the WeMos Arduino Zero Clone board.
+
+## Overview
+
+The WeMos Zero is a cheap clone of Arduino Zero featuring a ATSAMD21G18A.
+Unlike the original Zero, it does not include an embedded debugger/programmer.
+Instead, the [UF2 Bootloader](https://github.com/microsoft/uf2-samdx1) is
+used to program this board. The SAMD21 is a ARM Cortex-M0+ micro-controller.
+It has 256KiB of flash memory and 32KiB of RAM.
+
+Note: The official Arduino Zero using the native USB port and the Adafruit
+Metro M0 Express should also work with board definition.
+
+## Hardware
+
+![wemos-zero](https://forum.arduino.cc/index.php?action=dlattach;topic=526753.0;attach=244160)
+
+
+### MCU
+| MCU           | ATSAMD21G18A                          |
+|:------------- |:------------------------------------- |
+| Family        | ARM Cortex-M0+                        |
+| Vendor        | Atmel                                 |
+| RAM           | 32KiB                                 |
+| Flash         | 256KiB                                |
+| Frequency     | up to 48MHz                           |
+| FPU           | no                                    |
+| Timers        | 6 (1x 16-bit, 2x 24-bit, 3x 32-bit)   |
+| ADCs          | 6x 12-bit channels)                   |
+| UARTs         | 2                                     |
+| SPIs          | max 6 (see UART)                      |
+| I2Cs          | max 6 (see UART)                      |
+| Vcc           | 1.8V - 3.6V                           |
+| Datasheet     | [Datasheet](http://ww1.microchip.com/downloads/en/DeviceDoc/SAM_D21_DA1_Family_Data%20Sheet_DS40001882E.pdf) |
+
+### User Interface
+
+1 LED:
+
+| Device | PIN  |
+|:------ |:---- |
+| LED0   | PA17 |
+
+
+## Flashing the device
+
+Assuming the [UF2 Bootloader](https://github.com/microsoft/uf2-samdx1) is
+installed, just run:
+
+```
+make BOARD=wemos-zero flash
+```
+
+Hint: You can you press the reset button twice to manually enter the bootloader.
+This is helpful if the board has crashed (e.g. due to a failed assert() during
+start up).
+
+ */