Introduce a Makefile for the OS_NONE device example

It currently supports the SAMD21 and SAMD51 only. More will be
added later.

hw/bsp/metro_m0_express/board.mk

@@ -0,0 +1,35 @@
+CFLAGS = \
+	-DCFG_TUSB_MCU=OPT_MCU_SAMD21 \
+	-DCONF_DFLL_OVERWRITE_CALIBRATION=0 \
+	-D__SAMD21G18A__ \
+	-mthumb \
+	-mabi=aapcs-linux \
+	-mcpu=cortex-m0plus \
+	-msoft-float \
+	-mfloat-abi=soft
+
+# All source paths should be relative to the top level.
+LD_FILE = hw/bsp/metro_m0_express/samd21g18a_flash.ld
+
+LD_FLAGS += -mthumb -mcpu=cortex-m0plus
+
+SRC_C += \
+	hw/mcu/microchip/samd/asf4/samd21/gcc/gcc/startup_samd21.c \
+	hw/mcu/microchip/samd/asf4/samd21/gcc/system_samd21.c \
+	hw/mcu/microchip/samd/asf4/samd21/hpl/gclk/hpl_gclk.c \
+	hw/mcu/microchip/samd/asf4/samd21/hpl/pm/hpl_pm.c \
+	hw/mcu/microchip/samd/asf4/samd21/hpl/sysctrl/hpl_sysctrl.c
+
+INC += \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/ \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/config \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/hal/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/hal/utils/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/hpl/pm/ \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/hpl/port \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/hri \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd21/CMSIS/Include
+
+VENDOR = microchip
+CHIP_FAMILY = samd21