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_m4_express/board.mk

@@ -0,0 +1,34 @@
+CFLAGS = \
+	-DCFG_TUSB_MCU=OPT_MCU_SAMD51 \
+	-D__SAMD51J19A__ \
+	-mthumb \
+	-mabi=aapcs-linux \
+	-mcpu=cortex-m4 \
+	-mfloat-abi=hard \
+	-mfpu=fpv4-sp-d16
+
+# All source paths should be relative to the top level.
+LD_FILE = hw/bsp/metro_m4_express/samd51g19a_flash.ld
+
+LDFLAGS += -mthumb -mcpu=cortex-m4
+
+SRC_C += \
+	hw/mcu/microchip/samd/asf4/samd51/gcc/gcc/startup_samd51.c \
+	hw/mcu/microchip/samd/asf4/samd51/gcc/system_samd51.c \
+	hw/mcu/microchip/samd/asf4/samd51/hpl/gclk/hpl_gclk.c \
+	hw/mcu/microchip/samd/asf4/samd51/hpl/mclk/hpl_mclk.c \
+	hw/mcu/microchip/samd/asf4/samd51/hpl/osc32kctrl/hpl_osc32kctrl.c \
+	hw/mcu/microchip/samd/asf4/samd51/hpl/oscctrl/hpl_oscctrl.c
+
+INC += \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/ \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/config \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/hal/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/hal/utils/include \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/hpl/port \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/hri \
+	-I$(TOP)/hw/mcu/microchip/samd/asf4/samd51/CMSIS/Include
+
+VENDOR = microchip
+CHIP_FAMILY = samd51