Add Stm32F7xxdisco board support files

Status with examples/device/cdc_msc:
- make BOARD=stm32f723disco        => OK
- make BOARD=stm32f723disco PORT=1 => No Reaction
- make BOARD=stm32f746disco        => OK
- make BOARD=stm32f746disco PORT=1 => Hangs during enumeration
- make BOARD=stm32f769disco        => Hangs during enumeration

hw/bsp/stm32f769disco/board.mk

@@ -0,0 +1,51 @@
+CFLAGS += \
+  -flto \
+  -mthumb \
+  -mabi=aapcs \
+  -mcpu=cortex-m7 \
+  -mfloat-abi=hard \
+  -mfpu=fpv5-d16 \
+  -nostdlib -nostartfiles \
+  -DSTM32F769xx \
+  -DHSE_VALUE=25000000 \
+  -DCFG_TUSB_MCU=OPT_MCU_STM32F7 \
+  -DBOARD_DEVICE_RHPORT_NUM=1 \
+  -DBOARD_DEVICE_RHPORT_SPEED=OPT_MODE_HIGH_SPEED
+
+# suppress warning caused by vendor mcu driver
+CFLAGS += -Wno-error=cast-align -Wno-error=shadow
+
+ST_HAL_DRIVER = hw/mcu/st/st_driver/STM32F7xx_HAL_Driver
+ST_CMSIS = hw/mcu/st/st_driver/CMSIS/Device/ST/STM32F7xx
+
+# All source paths should be relative to the top level.
+LD_FILE = hw/bsp/$(BOARD)/STM32F769ZITx_FLASH.ld
+
+SRC_C += \
+	$(ST_CMSIS)/Source/Templates/system_stm32f7xx.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_cortex.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_rcc.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_rcc_ex.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_gpio.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_uart.c \
+	$(ST_HAL_DRIVER)/Src/stm32f7xx_hal_pwr_ex.c
+
+SRC_S += \
+	$(ST_CMSIS)/Source/Templates/gcc/startup_stm32f769xx.s
+
+INC += \
+	$(TOP)/hw/mcu/st/st_driver/CMSIS/Include \
+	$(TOP)/$(ST_CMSIS)/Include \
+	$(TOP)/$(ST_HAL_DRIVER)/Inc \
+	$(TOP)/hw/bsp/$(BOARD)
+
+# For TinyUSB port source
+VENDOR = st
+CHIP_FAMILY = synopsys
+
+# For freeRTOS port source
+FREERTOS_PORT = ARM_CM7/r0p1
+
+# flash target using on-board stlink
+flash: flash-stlink